I'd say the bundles on OCUK are pretty good mate, then all you'd need (if you don't have already) is a case, HDD, CD drives and PSU and a OEM copy of Windows.
Something like this maybe?
Click me.
Ooops just read you'll be sticking with an Athlon, what one will you be going for? Like I said I've got an AM2 64 X2 4600 and its seriously crap but its been discontinued on a lot of sites and now all they sell is the 5200, 5400, 5600 and 6000 - all of which are CHEAP! The 6000 has had some good reviews too.
Oh and if you're just changing CPU you will not have to reinstall Windows bud.