I got a few suggestions when buying PC components:
- NEVER buy around christmas

hehe, since prices almost always get a drop in january / february
- don't bother with "I do not need this/that"... because you will come to a point where you will punch yourself in the face for NOT buying it.
1. If you plan on building a totally new system. I strongly suggest for future expansion to go for a 64 Bit AMD system. Although you have to be careful to chose the right Socket Type (since some of the coming processors will not fit into the same socket its 754 and 974 , IIRC).. I could do some research if you want.
2. Go for a motherboard that includes PCI Express Slot. Sure, at the moment you get almost the same FPS on AGP that you get on PCIe BUT the best cards when it comes to Preis/Leistung are the Geforce 6600 GT on PCI-Express ~160 Euros you can get one.
this one is good enough for most of the games out there. It will run fine with HZ, and with WoW anyway. It's good for Doom. I prefer NVIDIA for the drivers.. ATI run good with selected games and Windows generally... but NVIDIA is better at compatibiliy with games... you know about the ATI bug in HZ.
Check out some test of the 6600 GT versions. you'll be amazed.
3. You talked about RAM. 1024 is not only good but I think needed for good gaming experience nowadays. But you can rely on good brand modules from Kingston e.g.. It does not have to be Corsair, etc... if you do not plan on overclocking (I think overclocking is nice, but the little extra performance generally does not justify the work and money).
Just do not by "no-name"-memory
4. Do not underestimate the harddrive. When it comes ot scanning my HZ directory my Laptop does a better job than my desktop drive (dunno why). So a good Harddrive is VERY VERY important. Even for games like HZ which swap and access your drive a lot. The Samsung Spinpoint series are very nice.
5. As I said a 64 BIT system is not much different then a 32 BIT processor based when it comes to processor costs... So go for 64 Bit.
The OS version (64 Bit Windows) is still not that close to release, and frankly... it does not give much sense since most games are not coded this way... but some give patches. So it#äs better.
6. It's not legal to run ONE copy of XP on TWO systems. You would have to purchase another copy if you want to be on the safe side.
The difference between Professional and Home edition are some better administerung tools in the professional edition. But if you don't play around much you might not really need it and spend the extra cash.
7. Sound... well. Generally extra cards.. specifically the SOundblaster Audigy Series are better when it comes to sound. BUT some of the onboard chips are not performing that bad.
I suggest you get a board with onboard sound (basically all these days have that) and test it prior to buying a external card.
If you do not have a 5.1 or 7.1 system connected you might just do fine with the onboard sound.