If you're buying a processor for a gaming system today, Intel's Core i5-2500K (Sandy Bridge) CPU provides the best bang for your buck. If you're willing to wait another two months, you might consider a Core i5-3570K (Ivy Bridge) CPU, instead.
Previous-generation graphics cards like Radeon HD6950 and GeForce GTX570 provide good gaming performance, but the current-generation Radeon HD7950 is appealing because it is fabricated on a newer 28nm node, allowing it to use less power than similarly-performing GPUs of the previous two generations that were fabricated on TSMC's 40nm process. Radeon HD7870 should appear by the time that Ivy Bridge CPUs are available, filling the Radeon HD6950's performance niche.
i7-2600K, H70, P8P67-M Pro, 16 GiB, HD7950, SSD, 3 HD, Blu-ray, X-Fi Ti Pro, Antec P182, S75CF, 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD