ASUS M2N-E motherboard
G.Skill 4 gb (2x2g gb) 240-pin ddr2 800
AMD athlon x2 6000+ 3.1 GHZ
KEEPING video card the dsame sapphire HD4850
Would I notice a substantial improvement in gaming call of duty modern warfare 2 etc and the like.
Would it be worth the 300 bucks?
Would I need to swap out my power supply which is a Corsair 550W. (doubt it??)
If you manage to see an improvement, it will be very subtle. Since you're a gamer, consider selling your 4850 instead and replacing it with a 5850. For sure, you will see more of an improvement than your original proposal.
Sure. Upgrade the whole thing if you feel like selling your current rig. I was thinking you sounded "value-minded" and wanted to know how best to spend a little (or a lot of) cash to improve gaming performance.
Too many people are upgrading, spending huge cash, and in the end the performance gain that they can see (maybe 10%) makes them feel unsatisfied. I've been there, done that, and now when I upgrade, I get rid of the old platform completely and go with the latest, ie from 775 to i5 or i7 which I haven't done yet.
If you upgrade the GPU and overclock your current CPU, there won't be any games out there that will stutter on your rig. You can always upgrade the CPU, mobo, and RAM later if you think yo can't enjoy the current games due to slow framerates.