Will it make big difference? I am using dual core now. Sometimes can't change limit order because of computer lag.
I upgraded from a dual core chip to the i7 950 chip with windows 7 and 6 gigs of ram. The difference is incredible. It feels about 6 times faster running esignal.
If you are buying a new machine Intel "Sandy Bridge" is a no brainer: http://en.wikipedia.org/wiki/Sandy_Bridge i5 2500 or 2500K gives great performance for the money. No point in going for the older i5's or i7's.
Depends on what you are doing for trading. If you are trading manually half a dozen of futures, it won't make much difference (unless the trading software is particularly demanding). If you are running a real-time market scanner on the whole of NASDAQ, a faster computer has a potential to make a world of difference.
I remember there is a way to check how many cores are being in use. For example, if you use i5, you should see 4 cores. But sometimes, it just shows less than 4...