you really need to upgrade the MB,CPU,RAM and GPU's. to get to where you can really game with decent settings.
The 8400 Wolfdale was a great gaming CPU, but I think its nearing the end of it lifecycle.
$350 is a tight budget to do all that. a AMD x4 965BE might be possible on that budget, but I don't think it wouldn't be a big enough performance gain. and i5 Sandybridge is the direction you would go for good true upgrade but that is ~$350 by itself. so basically you would need around $500-550 for good upgrade
that being said, a good GPU would be the best bag for the buck - HD 6870 or a HD 7850 and then save whats left over for a CPU/RAM/MB upgrade later, maybe after we see what Ivy bridge brings to the gaming table.
the dual core processor isn't as good as some people would have you think. It'll play but you can't add cores. The video card sucks. This would be the only thing I would spend money on. I'd over do it as much as possible in hopes of moving it to another build in the near future.
Bad idea AMD right now. Extra cores but yours and that lack the performance of today's new stuff. Waste of money, waste even dropping a quad in yours.
Don't need more than 4gigs of memory. Another waste of money.
should have listed monitor/res. and stuff you do with machine.
Bah, just realized my motherboard doesn't support any more than a total of 4gb of ram. I'll just be upgrading my video card for now and hoping it makes the difference. I'm going to be using this for guild wars 2 and WoW. Current display is 23 inches with 1680x1050