Need Help Improving PC

[Solid-Stark]

Bear with me guys, I don't know too much about upgrading hardware. 

I have an HP a1510n and I'm looking to upgrade. Currently stock it runs an Athlon 64 3800+ 2.4GHz and has Nvidia 6150 LE integrated. As you can see graphics are outdated

Ive been looking around and would Athlon 64 X2 (Model: ADX6000IAA6CZ) fit on my current motherboard? It says its socket is AM2 and mine is 939. What confuses is that i read my motherboard (socket 939) supports Athlon 64 X2 series. If not, then what other processors are recommended? I know i will have to get a new cooling unit but will any universal work? And what about power supply? (current is 300w)

Also, I'm looking to improve the graphics but only plan to spend about $100; Lets say i would want to run Crysis or Gears.. I went to Best Buy website and is Nvidia 8400GS 512 DDR2 good? Or  how about ATI Radeon 4350 or 5450, Nvidia GeForce GT220? I dont get what the difference in memory does and why some are expensive or cheap. Last of all how do you compare an Nvidia to an ATI? (for example Nvidia 8400GS is equivalent to ___)

 

Thanks in advance.