The problem with most name brand computers is a poor graphic card, and that is more important than having the fastest new processor. I think Tom's Hardware did a review comparing the relative benefits of CPU upgrades vs. video and at the end of the day, the CPU was very marginal compared to the graphics card.
You are likely best off to decide what your budget is and see what that gets you from a local shop, unless you want to build your own. You may wish to look for a board that handles SLI or Crossifre configurations so that as video card prices come down, you can add a 2nd card to extend the life of your rig.
You can also see what the name brand guys have, but be sure there is an open pci-e slot and plan on upgrading the graphics card as part of the purchase price. The Radeon 4850 and 4870 were recently released at very aggresive prices, and are excellent cards. Last week NCIX had the 4850 on sale for $150. Benchmarks for these cards are all over the net.
Panasonic TH-42PX60U, Harmony 670, Rogers HD 8300, 3250HD, Denon AVR-591, Chartwell SL3-5A, Venturer SHD-7000, Samsung 1600 BR.