It doesnt matter if you go for Ati or Nvidia, both companys got great cards which will give you nice and smooth grafiks in GW.
What brand you choose depends on how much you want to pay for a new card.
And which slots you have avaible for exspanding.
Look here for more info about price compared to cards:
http://www.tomshardware.com/2007/04/...for_the_money/
Sis is kinda out of date for todays games, and I think GW supports DX9 now, so will defently look and run best with a DX9 card.
As a minimum for GW I would say either Nvidia 7600GT or Ati X1650 XT, both with 256 MB.
I have seen some Ati X1950 XT's with the fast gpu/ram, for a very low price lately here in europe, those are prolly the best performing cards for DX9 compared to theyr price atm.
Just check the charts
http://www23.tomshardware.com/graphics.html
And those are even avaible for AGP. If your mobo cant use AGP or PCI-E you might be better of getting a new pc. :/
Always compare ram and gpu frequenzy, plus ram bits and ns on the same cards made by different brands. Some sell cheaper because they are slower then others.
Plus some brands got bad reputation while others got a good reputation, you can always hit
www.guru3d.com and try to ask which brand to go for and which not.
Nvidia came out with the 8600 series now as well, but it will take some time before DX10 games are released, and ATI has not shown what they got yet, so might be best of to wait and see what hot when DX10 is running for full.
GW2 will most prolly come with full DX10 support, but thats still far off from now.