Most games are single-threaded, which means that a dual-core or dual-processor system will do very little to increase performance.
If you want to build a PC for gaming, my advice is to ignore the dual-core CPU's and go with one of AMD's FX processors.
The
AMD Athlon Athlon 64 FX CPU's are designed specifically for gaming.
Oblivion runs very nicely on my AMD Athlon 64 FX-55 processor, and the FX-60 and FX-62 are now available to improve performance even further.