Reviving my old Dimension 9100, upgrade GPU questions...

Hi,

I have just blown the dust off my old Dell Dimension 9100 and thought that I might aswell start to use it again... I have placed an order for 4x1gb of RAM for it and am going to buy 64-bit windows vista soon. Currently, it has: 512mb ram, 3.20GHz Processor Intel Pentium 4 HT 630 'Prescott' and a NVidia 6800.

I want to upgrade the graphics card first so I thought maybe a mid-range casual gaming card... I am giving my self a slightly flexible budget of £60 and I am happy to buy new or used on ebay or amazon etc.

I thought about buying a 512mb GDDR3 8600GT or a 512mb GDDR3 ATI 2900XT... I know the 2900XT is better but I was wondering how much better it was.

I have also heard that it is better to go for NVidia because so many of todays games are optomized for nvidia... Is this the case?

Could someone show me the best online Graphics card store in the UK?

Also, will the computer be able to handle the 2900XT or the 8600gt?

I would be greatful for any answers to my numerous questions and any other info I need to know before upgrading!

Thanks

MacBookFan