I vote NOD32 (version 4, at least). Memory footprint is very small. This AV gets good ratings and low false positives. Currently memory usage is around 59804 KB (unless I'm missing a NOD32 process).

Just get the AV, the "Internet Security" firewall isn't very good.

There are only four AVs in total I consider to be good enough for me to consider putting on a computer:

Paid: NOD32, Kaspersky

Free: Avast!, MSE

Of those, NOD32, Avast!, and MSE would all be fine on a gaming PC. Depending on your setup Avast! can use the least amount of memory (it's very modular), less than the other two. Though it also can use more memory than the other two if you go all out on the installation.

MSE doesn't support XP 64 bit.

*Sad face*