+1 for i5 rig.
core i7 dosent show any noticeable performance improvement over i5. yes threaded apps may give lil better performance. i5 is very VFM. Although going LGA 1366 would give you option for 6 six core in near future. ( If you want too upgrade your proccy within a year or so i.e.)
A good i7 920 rig will go over 50K with small improvement over i5 rig.