I7 = top dog, D0 stepping overclocks like a gun (2.6 to 4.2 on stock volts, some people even get that with
undervolting) X58 chipset is expensive and MB's have higher number of PCB layers then other chipsets (depending if the board has3X16 pci-e). tri channel memory. CPU Cost a lot as well.
I5 = intel mainsteam, can only do 1X16 pci-e, IPC is basiclly the same as I7. not sure on overclocking haven't really followed I5 much. Intel isn't pricing these as aggressively as they could depending what model some AMD's are better value/perf.
AMD phenom X4's = phenom 2 overclocks alot better then the orignal phenom on avg can reach 3-8. to 4.0, priced very well and motherboards with DDR3 as well as IGP with its own memory are very cheap. depending what the task is generally -10 to -20% IPC vs I5/I7
AMD phenom X2 550 BE = FUN
. some of these are X4's with locked cores, some are X4's with damaged cores, some are X4 with cores that are to leaky and some are a native X2 design. head over to SX they have a good thread tracking batch numbers, buy in the right batch and have a good chance at getting a quad core for the price of a dual. Because it is a black edition no locked multi. need to make sure you buy the right chip set, you need something with either SB710 or SB750 to unclock. same IPC as phenom X4
AMD athlon X4 = the phenom have L3 cache(6mb) the athlon doesn't. look at benchmarks closely depending on your workloads their might not be much of a performace difference or there could be a lot
ipc from 0 to about -10 of Phenom