i am building a computer and need all the money i can save on this but i dont want to sacrifice preformance too much so is it worth it to upgrade from a i5 650 quad core wit hyperthreading to a i7 870 quad core wit hyper threading
Why not an i5 750/760? Hyperthreading is not needed for 99% of users and for the record 4 core >> 2 core+Hyperthreading. If you really need it, get an Xeon X3440 which is a good middle ground.
I'd say no, it's not necessary but it really depends on what you're doing with the PC. Either way you could wait until Intel releases Sandy Bridge which might bring the prices of the current i7's down. Even then it would probably make sense to save up and get an SB setup.
the i5 650 is a dual core... i just replaced my i5 660 for a i7 870..and for the game i play FSX (the cpu killer) i saw a night and day preformance improvement