Only Vista 64 will recognize 4 gigs of RAM or more. Not even Vista 32 will recognize 4 gigs of RAM. The most XP will recognize is 3.5 and I am not sure if that is limited to the PRO version or not so I say stick with 2 gigs since it is XP and you should do fine.
I checked out your link:
Processor Intel Pentium CORE 2 DUO E6850 3.0GHz 4MB 1333FSB
Excellent!
Memory OCZ Gold XTC OCZ2G8002GK 2X1G DDR2 800Mhz
I personally would go with Crucial or Kingston in DDR2 800MHz if Asus recommends those manufacturers for this board. Asus boards are quite well known to be picky about the memory utilized. This is not to say they are poor quality motherboards -- far from it! Asus makes great boards.
I belong to several tech forums and one person whom I highly respect and who has been working in the field of building and repairing PCs/laptops for years just posted how OCZ quality greatly fluctuates because they use a lot of third party manufactures. For what it is worth this is what he posted: OCZ has only been in business a short time. They make very little of their own memory. The quality of OCZ memory swings more widely than most. OCZ is mainly a marketing company out of Singapore which bought out a small Canadian company then loaded up the market with good reviews... but a tremendous amoung of their memory now is third tier that changes from month to month. When they sell Value Ram and low density, the chips are not even the same from quarter to quarter. I recommend you avoid OCZ products at all times.
Get Kingston, Crucial, Infineon, Samsung, Toshiba, Corsair, Siemens... which all have several levels of quality and speed.
Mainboard ASUS P5K-Deluxe/WiFi-AP S775 P35 Chipset
Good choice. As I said, Asus makes good, solid motherboards. This model has slots for up to 8 gigs of memory, 6 USB slots)
Optical Drive Pioneer DVR-212D SATA Black 18x18 DVD-RW
Can't say because I don't know. If you ever really want to know the quality of an item I usually check out customer's reviews over at NewEgg.
Hard Drive Seagate 320GB SATA-II 16MB 7200rpm
Yes! Good selection, plenty of speed and it is a SATA II. And, DW, you'll have all that room for Morrowind Mods!
Video eVGA E-GEFORCE 8800GTS 320M DDR3 PCI-E
It is good you are going with the 8800GTS rather than the 8600 models and especially not the 8500 models; and it is DX10 ready.
Power Cooler Master RP-600-PCAR 600W eXtreme Power Supply
Make sure this is a good quality power supply. Just because it says 600w dowsn't mean it is going to give you 600w. Ask them what you will get on the 12v rails. The higher the better and the rails should be no less than a combined 30A.
FREE GAME! Might and Magic: Dark Messiah by Ubisoft (Limited Quantity)
If you don't want it can I have it? 
DW, you certainly did you homework and you should really be very fine for at least 2 years. Because you are still using XP your current games should run as they always have. Just make sure the drivers for the 8800 card are good because Nvidia's early Forceware drivers for their 8800s were causing nightmares.
Kaspersky is a great internet security system but keep in mind that in the beginning it will continously have popups asking if you want to do so and so, or permit so and so. How do I know? I just built a new system and installed KAS 6.0 and it does this as it learns your preferences.
You are more than ready for Vista when the time comes. Nice system!
