The processor of choice these days is the Intel E6600 or E6700, for
cheaper systems maybe a E6300. The E6300 is 1.86GHz, the E6700 is 2.66GHz,
The new Core2 Duos are faster then the Athlon 64 X2s which in turn are
much faster then the P4s.
The motherboards for the Athlon 64s offer a lot more for the money then
the motherboards for Intel processors. The Nvidia Nforce chipsets for the
A64 motherboards have built in gigabit MACs and built in RAID controllers.
The on chip MAC also has a TCP/IP offload engine. Generally the A64
motherboards have two Gigabit controllers and a couple of SATA-II RAID
controllers plus a couple of regular SATA-II connectors. The Intel
motherboards are all using external MACs and RAID controllers and they
generally only have one gigabit controller and one RAID controller. None
of this matters to you if all you want to do is play games.
The thing that does matter is SLI vs Crossfire. There are a lot of A64 SLI
motherboards, very few Crossfire A64 boards. There are a lot of Intel
boards with Crossfire support, there is no SLI support of the Core2 Duo
yet. All this is pretty ironic given AMDs purchase of ATI.
Nvidia graphics cards will work in any PCIe motherboard, but of course of
you want SLI you'll need an SLI capable board. Like wise ATI graphics card
will work in any PCIe motherboard under Windows (their Linux drivers suck
so Linux users like me generally stick to Nvidia which has excellent Linux
support). And of course if you want Crossfire then you'll need a Crossfire
capable board which means that you'll want an Intel system.