Dual Xeons, baby. 2.8 or 3.06ghz is where the sweet point is for price at the moment. The Opteron is kinda meant to compete with both the Xeons and the Itaniums. The 14x and 24x series are good for workstations, while the 24x and 84x series are good for servers. The Opteron 246 is a good performer, but is very very expensive ($800+ per chip), so I'd recommend a dual 3.06ghz/512k Xeon (less than $500 per chip) over it. The Xeons with an extra 1MB L3 cache probably aren't worth it, although they're definitely "cool".
With dual processors you can game and "crunch" at the same time: just the other day I was playing Deus Ex: Invisible War while encoding a DivX. No slowdown in the game at all, it was smooth.
For price, your best bet however is a single Pentium 4 with HyperThreading. They're a lot better for multitasking compared to a regular Pentium 4 or any single processor AMD solution.
With dual processors you can game and "crunch" at the same time: just the other day I was playing Deus Ex: Invisible War while encoding a DivX. No slowdown in the game at all, it was smooth.
For price, your best bet however is a single Pentium 4 with HyperThreading. They're a lot better for multitasking compared to a regular Pentium 4 or any single processor AMD solution.