As prebuilt machines can vary widely in what's inside even within the same model number, you'll really need to give more detail on the machines before we can give a useful recommendation.

at least tell what kind of CPU and graphic card you have, and how much RAM. If you don't know that, I suggest trying a software like Everest(http://www.lavalys.com/products.php?lang=en&page=11) that will tell you much

I had a look about the GX270, and I have some doubts about it being able to run GC2...the integrated graphics doesn't help.

Have a go at the demo if you want to be sure, but I'm 90% sure that it won't run decently.

If my informations are correct, you don't have an AGP slot on the motherboard, so you'd need to find one of those old cards that use PCI(*not* PCI-Express). That may be tricky...and you need at least a card with minimum 64MB of RAM to be ok. 128MB is better. Honestly I don't know much about this anymore, Google is your friend though

If you *do* have an AGP slot it's much easier...a Ati Radeon 3450 could be a nice economic choice that could even enable you to watch some movies on that PC. Just be sure to get the version that uses AGP and not PCI-E.