But Windows detects all the RAM (VRAM and Physical Memory too), it recognizes a maxed of 3.5 I believe, then it starts subtracting based off things like Video RAM.
I say keep your current cpu , get a good aftermarket heatsink/fan (Newegg.com - Sunbeam CR-CCTF 120 mm Core-Contact Freezer CPU Cooler W/TX-2 - CPU Fans & Heatsinks) and overclock the sh** out of it. You should be able to get 3.5-4 ghz out of that cpu. I'm not sure what kind of case you have , but the Sunbeam is pretty huge. They make a smaller version of the core contact freezer , but I don't know about it's effectiveness.
Word of warning : The heatsink that comes with the E5200 is junk. Don't try to overclock with it.