Killing Floor might be a good choice for you, and with the summer sale going on right now, you can probably snag it for cheap if it goes on sale again anytime soon. I mean, it's still a good deal for $10, but you might have a chance of getting it for $5 if you wait until the 23rd to see if it pops up again.
It's not too high-end to run I believe, basically could run it on Integrated graphics if you really pleased. It's similar to Black Ops in that you're killing waves of enemies (they're not technically zombies) but it has a set amount to complete the level. I'd say it's worth a quick check.