i currently own this pc
4gb ddr2 800
500 gb spinpoint f3 samsung
p5k se epu motherboard
i am pretty happy with this build, but in some games i get low framerates ( i run at maximum 1280x1020 because of my monitor ) - gears of war looks very good, but moves slow some times, crysis 2 has the same problem
i am thinking of changing the motherboard, processor and rams ( or should i change the video card too? )
i got a budget of around 300$
i was thinking of an amd x4 955 BE with an AM3 mb and 8gbs of ddr3 1333mhz
if i sell my video card i could get a budget of a total 400$
Here is a suggestion...make sure you have task manager running in the background when playing a game. Next time the game slows way down ALT-Tab out and look at the performance tab in task manager to see the history graphs of CPU usage. If during the low fps time your CPU is pegged at or near 100% then you probably need that upgraded (you need to alt tab pretty quick as task manager does not have much history). However, if the CPU is not pegged then on that game your graphics card is probably the bottleneck (this of course can vary by game). You can also get things to monitor GPU utilization to make sure it is hitting 100% when you see the game slow down.
Can't go wrong with the amd phenom II-X4 955 BE, future proofing yourself a bit - browse around newegg or tigerdirect for bundle deals, 8 gb ram should cost you around $40, CPU $120, MB $50 and a 6850 GPU for $139 (seen it lower on sale with newegg but has expired- try calling them) - total $349. Make sure your psu is at least 500W for the 6850.
This also gives you headroom for overclocking the 955 if you desire more speed. This leaves an extra $100 that you can spring for a 128 SSD for your boot-up and games while your other HDD as storage drive for your other data and infrequently used data. Now this a sweet set-up.
What is the make and model of the 350W unit that you have?
Does it have a PCIe connector?
I have read about someone using a 5850 (which is more power hungry than a 6850) with a ~300W PSU, but that was with a less power hungry 65W i3-500 CPU. So that points to a 350W PSU being enough. But it would definitely have to be a high quality unit that can easily provide its labelled wattage.