For what you might spend on a mobo+CPU+supply+other "stuff"... you might find a heck of a deal on what are called "Barebones" systems. These will typically have a case, power supply, mobo, CPU. After that, they might have memory pre-installed or not, usually they're missing a disk, CDROM/DVD, etc.

If I were upgrading a system today and the following were issues:

1. The power supply wasn't big enough.

2. I would need new memory in addition to the new CPU+mobo

I'd look really hard at a barebones system, get the RAM needed for the new CPU+mobo, and pull over all the disks, etc from my old system. Then, if I had some spare change, I'd go get some really cheep drives/CD's, etc and put 'em in the old system and have two usable systems.