The 8400 will get a 3dmark 06 rating of around a 1000 with that cpu. The 8800 will get around 7500 with that CPU. I have a intel core2 1.8 with an 8800gt and it runs pretty good. It runs MW2 on med, black ops and bad company 2 on low.
Tiger direct had a GTX250 for like 75 with a mail in rebate. I don't think you will get high settings but the system will run those games pretty well.