Honestly a new PC based on 64Bit Windows. Your system maxes out at 3GB (32bit can't "see" above 3GB) of RAM memory to 'think' how to do something. Second it is Core2Duo, these were made for the other 90% computer users out there, not for high end gaming. You would need at least a i5 (which may actually swap out into this PC, but you have to look it up with the motherboard to see if it supports the iCore CPUs).
The problem is when you start to piecemeal like that your costs add up more and more, when it be more 'cost effective' going forward to instead just swap out for a new PC, and reuse your 660 (this is a desktop right?) and swap the PSu in the new PC. For example I seen for only $599 a i7 Haswell from Lenovo, which is dirt cheap for the newest CPU type AND a i7 core at that. If you shave off $100 then your down to a i5, but for that $100 you get more performance and much longer life out of the system.
Just a thought to consider