Hm, well depending on your budget, you'd probably benefit from something with a quad core xeon (nehalem would help) maybe a 5504 (2ghz) or 5520 (~2.26ghz) 3 or 6gb ddr3 ram, depending on your budget. 4x250gb hdds in raid10.
That server will cover all of your needs plus more. Having this kind of setup will ensure that you will have data redunancy, very fast processing power due to the power of the nehalems, not to mention ddr3 ram and QPI.
You'll easily be able to cover for future expansion with this server.
Dual Quad core Xeons are pretty expensive setups, especially when you get into the nehalem range. All depends on the budget really.
If you make decent money from advertising (if you have any ads) then by all means get a dual cpu quad core xeon setup. Faster load times means more pages can be viewed in shorter time, which means more possibility for ads to be clicked or seen!