My system right now is an AMD single core 3500+, 1 GB RAM, and 256 BM Radeon x800.
I'm planning to do some upgrading and trying not to spend more than $400. I know I'm getting a monitor for about $150, a case with better airflow so my room isn't so hot, and a second hard drive.
I have a socket 939 and DDR400 so I was planning to just the fastest CPU Newegg has which is a 4000+ and adding 2 GB of RAM. If I was going to get an AM2 I would have to get new RAM which I'm not sure if I would be spending more that way or not.
I haven't kept up with video cards so I don't know what a decent mid level card is anymore for a low cost upgrade in that area.
If you get an AM2 you will also need a new motherboard Decent Mid Range, go for either an 8800GT, or a 9600GT, both are cheap now, and offer mediocre performance
Radeon HD 4850. You would be nuts to get anything else at this point.
The 4850's outperform the 8800's by a good bit, and beat the 9800GTX as well, all while only costing $200, or less with the mail in rebates offered on many.
If your willing to spend more, the Radeon HD 4870 is the best bang for you buck card you can get. With a 4870, your getting near GTX 280 performance for only $300. Though there would probably be a bit of bottlenecking with your 3500+.
I have a socket 939 and DDR400 so I was planning to just the fastest CPU Newegg has which is a 4000+ and adding 2 GB of RAM.
It might be a better idea to look around for a S939 Athlon 64 X2 at this point, if you can still find one that is. A 4000+ might be a decent upgrade, but your still going to be stuck with a single core CPU.
The 4850 is a really good bet right now if you can afford it. If you must buy now but can't afford the 200 then the 8800GT or HD 3870 will also serve you well. I've seen the 3870 going for 120 on newegg, without rebates. Personally I would try to shell out the extra cash for the 4850.