First thing why have you gone for a 64 bit system, if it's because of memory problems forget it, Vista 32 bit will recognize 4GB of memory with no problem.
Also why go for the quad core an Intel Duel core E 8400 would be a better chip, it's a more up to date chip that runs a 3GHZ and it runs cooler.
Also find out about the PSU, what make is it.
Graphic card, you would be better going for the 8800 GTS, it's not much dearer but a lot better card.
The warranty is a little bit dodgy, first month only paid by the firm, you could find that any savings made could be wiped out if you have to return it after that time, it can be up to £50 to return a p/c and if that happens a couple of times it soon mounts up.
This part I'm not to sure about but some games will just not work under Vista and if you have any older ones you could find they not working.
Go for Windows XP at least then you know all the other hardware you have will work with it and even XP will accept 4GB memory, I've got the same memory in my p/c and it's fine, it only shows 3.2 GB of memory but it works well.