ATI RADEON X700

Any good at what?

Its ok, I had one in my laptop, run most games out there with a few notable exceptions of those that require SM3.0 support.

While pretty equal in the past, nowadays I'd recommend you get a GF6600GT instead or an X1300XT or GF7300GT or HD2400 or GF8500 depending on your price range and type (agp you're a little more limited).

Will it play big games? I know it's a bit old now (came out in late 05 / early 06) but it was considered good then I believe.

Basically, I've seen a pc that has nice specs that I might buy but I'm not sure what games it would play and how well it would play them.

Tell us what are the full specs of the PC, and price, and well tell you if it's a good deal.

They will be ok for most stuff.
It will at least 'play' crysis on LOW settings, but unfortunately some games like Rainbow6 Vegas, etc it won't play at all due to the requirement of an SM3.0 compliant card.

It's solid , but I'd recommend the other options I listed if you want to play 'big games' (ie Crysis, Bioshock, COH, COD4, etc).

If you're lucky and the PC has PCIe slots, you canget a much better gaming card for about $60 on NewEgg.