Upgrading My Budget Machine

The fx-55 is much more than a 600mhz boost, it will wipe the floor with your 3000+. For you though, a graphics card update will do you much better than a cpu update. Overclocking is a very good way to get more performance. You won't damage your parts by overclocking, if you do you are the most unlucky person in the world.

I'd say get this motherboard http://www.newegg.com/Product/Product.asp?Item=N82E16813127220
and this videocard http://www.newegg.com/Product/Product.asp?Item=N82E16814102688.

Then overclock your cpu. There are plenty of guides out there on how to safely do this. My only concern is that 350w is not enough power for stable operation with an overlclock or with the new graphics card. Get this power supply if you are experiencing BSoD's or very bad performance in games http://www.newegg.com/Product/Product.asp?Item=N82E16817153023. If you wait long enough there will be a 10$ mail in rebate for that PSU. They have been doing that for at least a year on that PSU.

Your GPU will most likely be a bottleneck for an FX55. 7600GS is a weak GPU compared to the x850xt. If you get the motherboard and the gpu I reccommended you will have incredibly faster FPS.

As for your memory question, yes the ram will run with 2x512 and 2x256. It will run in 2t mode. If you use 4 sticks of ram you switch to 2t wich is slower than 1t mode with just one or two sticks of ram. It depends on the games you are running, some will show worse performance with the 1.5gb due to the 2t latency and some will show benefits due to the fact you have more ram.

You have to match identical sticks of ram (or nearly) to do so, your motherboard might give you a fit if you run two seperate sticks of ram in dual channel mode. So 2x512mb corsair and 2x256mb g.skill might not work or cause instability. Either way you should try it, it's not like you will hurt anything by it not posting or crashing at the windows screen.