I have this in an old P4 machine and it runs WoW with no problems at all, $31 at amazon. Keep in mind your not going to be playing anything like BF3 or GW2 with this thing but I have played been playing EQ/FO3 and GW for a few years on it.
If you might play other games down the road then the 6670 is something you should be looking at as far as affordable cards go. As somebody already posted they can be had for around $60 after rebates. Personally I would be looking at the 7750 or even 7770 if you have a 6pin on your power supply better bang for the buck over the 6670 cards right now.