Have a look at the http://ppbm4.com/Benchmark.html results to see what makes a fast computer. If you do the test yourself, you will know where you are currently, but my guess is that you will end up with a total score around 130 - 150 seconds.
Also have a look here: http://forums.adobe.com/thread/598040
Keep in mind that budget is good for DV & HDV and for AVCHD you need to look at the economical build.
You don't say the brand/model of your motherboard, but looking at what you have and GUESSING, I'd say you would need a new motherboard to use an Intel i7
With what you MAY have, a quad-core processor would give some increase
But still like a band-aid. They stick for a week maximum. The OP needs a new system, not a half baked solution.
If one uses AVCHD, one has to get a new PC as well, at least very often. The OP can still use his disk and video card in a new system, only the mobo, CPU and memory need to be replaced and the OS upgraded to Win7. Pretty easy.