As far as CPUs go, the only main thing the i7 has over the i5 besides being clocked slightly faster is having intel's dual-pipeline technology, called hyperthreading (and maybe 2 more cores, I forget). It's only really useful if you do things routinely to max out your CPU and those applications are multithreaded. (Video encoding, Photoshop, any (decent) movie making software, etc.)
Basically, unless you can through a lot of multithreaded work at it, go for the i5.
Edit: Also ensure the power supply you're choosing is up to the task. It's somewhat tricky, but just remember that pretty much everytihng besides the motherboard uses the 12V power line, so those amount of amps that the PSU can deliver on that 'rail' is important to know.
Basically, unless you can through a lot of multithreaded work at it, go for the i5.
Edit: Also ensure the power supply you're choosing is up to the task. It's somewhat tricky, but just remember that pretty much everytihng besides the motherboard uses the 12V power line, so those amount of amps that the PSU can deliver on that 'rail' is important to know.