If it was me i would do the following (sorry if someone else has mentioned the same):
For now, buy the 4670. Despite whats being said by some it will give you a huge boost and you'll be able to play current and maybe a few future titles, albeit at medium/low settings. There will be a bottleneck but it wont be so much that it makes things unbearable. (i've seen vids of COD4 being played on an atom/ion chipset,so trust me the cpu isnt *that* much of an issue with the exception of a couple of games)
Then, save up and buy a second hand am2+/am3 mobo, and a cheap AMD dual core. You'll have the ability to overclock this to pretty high speeds, and before you know it you'll have a very capable machine. Theres no problem in using the PSU/Case/hdd from your old system for now. Hope this helps, it's pretty much the same i thing i did afew years back, except my first gpu was a 7300gs