Based on the benchmarks of the Starcraft 2 beta (for example: http://www.legionhardware.com/articles_pages/starcraft_ii_wings_of_liberty_beta_performance & PC Games Hardware (PCGH): Computer, IT Gaming, PC-Spiele)
and various other generic benchmarks of CPUs and graphics cards I’ve concluded the following:
Starcraft 2 uses 2 cores, more or less, a quad core (at similiar speed) will only increase performance by aproximately 10-20%. This, however, may change, possibly in the expansions later on. It is possible that more than 2 cores becomes important in very demanding situations such as those that might ocur in custom maps.
Starcraft 2 is very much CPU-heavy and a good dual core CPU is therefore of supreme importance, thus:
The powerful and expensive graphics cards are of less importance, a midrange graphics card will do fine.
CPU: i3-530 $125
This is CPU should be almost as fast as the most expensive models like the i7 870 and is much, much cheaper.
Motherboard: GIGABYTE GA-H55M-USB3 $110 ($10 MIR)
Decent motherboard with USB 3.0, nothing much to say, most compatible motherboards should do fine
Case & PSU: Antec Sonata III 500 with 500W PSU (EarthWatts 500) $110 (20 MIR)
A decent case with good airflow is always nice to have to prevent any overheating mishaps, the PSU is suprisingly good too, and this combo is good bang for the buck.
Graphics Card: MSI R5770-PM2D1G-OC Radeon HD 5770 1GB (or any Radeon 5770 card) $160 ($10 MIR)
The 5770 is more than plenty, has low power consumption and has Directx 10.1 and 11 too.
It’s really the only card I would consider in this case. It can run Starcraft 2 (beta) in 1920x1080 at the highest settings, and it’s fairly cheap.
RAM: Crucial 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1066 $97
2 GB is too little, 4 GB should be enough, can always be upgraded later to 8 GB.
Hard Drive: SAMSUNG Spinpoint F3 HD103SJ 1TB $90
Well any harddrive will do, a slightly slower harddrive will increase the loading time, but little else.
Total: $652 (with MIRs)
If you want to reduce the price a bit you might be able to use your current PSU and hard drive, but without any more information, it’s hard to say.
if you desire not to build your own system I would recommend buying a system with an Intel i3 orIntel i5 CPU, they should offer the best performance-value by far in Starcraft 2. You could do with less, like the Phenom II X4, but you said you wanted something that runs Starcraft 2 really, really well.
I would NOT recommend this computer. As I said previously Starcraft 2 is very CPU-heavy and the Phenom X4 9650 is just not good enough. You need something like the Intel i3, Intel i5, Intel i7 or the Phenom II X4 965, but the i3 will probably provide the best value and should perform almost as good as the most expensive CPUs.
I usually wouldn’t recommend buying a premade system though, they tend to skimp on things like PSU, cases and graphics cards, and it really only takes about 2 hours to build your own and it’s not that hard either.
If you must buy a premade system go for one with one of the aforementioned CPUs, and buy the Radeon 5770 graphics card seperately.
If playing star craft 2 is really your only objective, and you’re not interested in future proofing your PC, then an older generation Q6600 or Q9550 intel CPU or an Athalon 64 x2 AMD CPU with 2 gigs of RAM and an Nvidia GTX 260 or HD 4750 will fit the bill for a good $150-$200 less.