Different colored RAM slots.

Yes, your motherboard supports up to 4GB of DDR 333 or 400MHz:
http://www.ldlc.com/fiche/PB00027816.html
however, I can't find your motherboard on the Asus website; it is likely a special manufacturing for the brand of your PC
so I can't tell you what is said in the manual regarding memory slot filling

the best way to get an additional 1GB is to add 2 x 512MB of DDR like the sticks you already have, because this way you will be in Dual Channel, which is a bit faster, especially with a Pentium 4.

Hello,

Yes, colors matter; otherwise, there would only be one color ;)
Read the manual for your PC or motherboard; it's explained:
The filling of the slots can't usually be done just any way,
and it depends if you want the RAM to run in Single or Dual Channel.
On certain motherboards, some configurations are not possible,
but we don't know your motherboard...
Please always provide your specs if you want us to help you effectively ;)

Normally, the color coding allows you to identify the pairs of slots for dual channel operation.
If you have two RAM sticks, you need to install them in the slots of the same color to benefit from dual channel.
Dual channel typically requires RAM sticks of the same size.
However, you can choose not to use dual channel; in that case, you can place the RAM sticks however you like.
It's still best to check the manual for your motherboard.
--
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Here is my configuration :

Field Value
Computer
DirectX 4.09.00.0904 (DirectX 9.0c)
Internet Explorer 7.0.5730.11 (IE 7.0)
User name Rémi
System name PC-PLOTARD (Marc)
System Service Pack [ TRIAL VERSION ]
Operating system Microsoft Windows XP Home Edition
System type ACPI multiprocessor PC
Connection domain [ TRIAL VERSION ]
Date / Time 2009-03-08 / 15:32

Motherboard
Processor type Intel Pentium 4 630, 3000 MHz (15 x 200)
Motherboard name Asus P5RD1-V (3 PCI, 3 PCI-E x1, 1 PCI-E x16, 4 DDR DIMM, Audio, Video, Gigabit LAN)
Motherboard chipset ATI Radeon Xpress 200
System memory [ TRIAL VERSION ]
BIOS type AMI (10/25/05)
Communication port Communication port (COM1)
Communication port Printer port (LPT1)

Monitor
Video card Radeon X1300XT/X1600Pro/X1650 Series Secondary (256 MB)
Video card Radeon X1300XT/X1600Pro/X1650 Series Secondary (256 MB)
3D accelerator ATI Radeon X1600 Pro / X1300 XT (RV530)
Monitor ImageQuest Q770 [17" CRT] (154220051017)

Multimedia
Audio card Realtek ALC880(D) @ ULi/ALi M5461 High Definition Audio Controller

Storage
IDE controller IDE PCI Bus Master ALi M5229
Storage controller ULi SATA Controller (M1573)
Floppy drive Floppy drive
Hard drive ST312082 7AS SCSI Disk Device (120 GB, 7200 RPM, SATA)
Optical drive LITE-ON DVDRW LH-18A1P (DVD+R9:8x, DVD-R9:8x, DVD+RW:18x/8x, DVD-RW:18x/6x, DVD-RAM:12x, DVD-ROM:16x, CD:48x/32x/48x DVD+RW/DVD-RW/DVD-RAM)
Hard disk status SMART Unknown

Partitions
C: (NTFS) [ TRIAL VERSION ]
Total size [ TRIAL VERSION ]

Input
Keyboard Standard 101/102 keys or Microsoft Natural Keyboard PS/2
Mouse Microsoft PS/2 Mouse
Game controllers Microsoft PC joystick driver

Network
Main IP address [ TRIAL VERSION ]
Main MAC address 00-15-F2-33-96-F0
Network card Marvell Yukon 88E8001/8003/8010 PCI Gigabit Ethernet Controller (192. [ TRIAL VERSION ])
Modem Communication cable between two computers

Devices
Printer HP Deskjet 5900 Series
USB Controller ULi/ALi M5237 USB Open Host Controller
USB Controller ULi/ALi M5237 USB Open Host Controller
USB Controller ULi/ALi M5237 USB Open Host Controller
USB Controller ULi/ALi M5273 USB 2.0 Host Controller

DMI
BIOS vendor American Megatrends Inc.
BIOS version 0907
System manufacturer System manufacturer
System name System Product Name
System version System Version
System serial number [ TRIAL VERSION ]
System UUID [ TRIAL VERSION ]
Motherboard manufacturer ASUSTeK Computer INC.
Motherboard name P5RD1-V
Motherboard version Rev 1.xx
Motherboard serial number [ TRIAL VERSION ]
Chassis manufacturer Chassis Manufacture
Chassis version Chassis Version
Chassis serial number [ TRIAL VERSION ]
Chassis identifier [ TRIAL VERSION ]
Chassis type Desktop Case
Memory sockets (Total/Free) 4 / 2

Problems and suggestions
Problem Free space on C: is only 1%.

Otherwise, I will definitely consult the manual, at least I will be set!
How is dual channel more advantageous? And can I do a "double" dual-channel? That is to say 2x512 and 2x1024, or even 4x512?

Yes, you can add 2 sticks of 512MB as long as they are the same DDR, DDR2, or DDR3 and have the same frequency.

you can put up to 4GB of DIMM DDR 333/400 MHz

Great! Thanks everyone for your replies! I’ll be able to manage for less than €20! Would this be suitable for my PC: http://www.2xmoinscher.com/info/detail.asp?id=345237?
Otherwise, I did indeed find something about the slots in the book (in English obviously) for my motherboard. Only, I'm not sure I understand the table, so I’m attaching a small photo: http://servimg.com/image_preview.php?i=95&u=11032245

There you go! And by the way, what is the maximum amount of memory my motherboard could support?

Thanks again for your replies!

Hello,

Yes, this RAM is generally suitable for your PC.
Your PC accepts one stick in any slot, or two sticks in the same color, or four sticks.
You already have two 512MB sticks installed in the blue slots, for example.
You just need to add two 512MB sticks purchased for the black slots,
and then you'll have 2GB of RAM in Dual Channel, which is good for XP.

Since you can put up to 1GB per slot (max RAM: 4GB, message 6), you could buy two 1GB sticks for a total of 3GB, still in Dual Channel, which is perfect and very close to the maximum useful possible.

Indeed, the maximum RAM had been specified by Marcmarais, sorry I didn’t pick up on that. For the 3 GB, I still think I'll settle for buying 2x512 MB, because generally I never run more than one heavy application at a time (a game for example, but nothing in the background). That said, with games requiring more and more power (1 GB minimum for Empire: Total War), I might invest someday in 1 GB sticks...

In any case, thanks to everyone for your answers, it’s really nice of you to take the time to reply!!!

You're welcome, keep us updated when you get your RAM sticks!