kincaid88 :
Onus :
Get the X3 450 then. The extra core will make a visible difference. Your 300W PSU can handle it, and a HD7750.
So it most definitely can handle the X3 450, HD7750, and 4GB more of RAM without upgrading to a bigger PSU, correct?
Theoretically yes, your PSU should be more then adequate to handle the Athlon x3 450 + an HD 7750, however, all this will depend on the quality of your PSU. Based on the spec you provided for your OEM system (Asus Essentio CM1630), ASUS had the "peak" wattage at 350W for the PSU, so from that you should have more than enough.
As for upgrading your RAM, look at the QVL (Qualified Vendors Lists) straight from ASUS, for your mobo: http://dlcdnet.asus.com/pub/ASUS/mb/socketAM3/M4A78LT-M/AMD-DDR3-4DIMM-Memory-QVl.pdf, to match your Samsung memory to avoid hardware conflicts.