CPUs - Multiple multicore processors are best. You will find that most mid-range workstations are configured like this. If that costs too much, go for a multicore processor. Single-core processors these days are placed under the budget category and they are basically all old architectures designed three or so years ago.

Hard drives - For performance a couple of 10 or 15K rpm SAS drives in a RAID array is best, but it would be very expensive and maybe even impractical to have such an array. SAS drives cost more, run hotter and have much less capacity than SATA drives as they are targeted for performance. A fast and smaller SAS system/currently-work drive/array and a slower and bigger SATA storage/array may be a good idea.

Graphics - Even though I like ATI (actually, just Ruby), NVIDIA is generally faster, although I've seen ATI win in a few benchmarks that probaly doesn't even apply to Maya (they are for games). Pro cards like the Quadro and FireGL have different firmware for better OpenGL performance and image quality and are much more expensive. Unless you need certified hardware support, as businesses do, GeForce or Radeon would do, although there is a tiny chance that the drivers might not work 100% correctly with Maya. A note on ATI cards at the moment, it seems that recently all of them have driver problems with Maya. The current drivers should have resolve this.

Since you are going with Linux, do some research into what the drivers support. Linux is known to be very picky with wireless Ethernet. Support for graphics is also a bit messy - gamer cards are not so well supported, but the professional ones are. Also, get a distro that doesn't change every six months into something unrecognisable and has lots of frequent updates, eg. Fedora - as changes to the kernel may render the drivers for the graphics card incompatible since NVIDIA and ATI take ages to update them.

If you are using the system for work, it might make sense to check out the more stable distros (CentOS, a RHEL clone or Debian) or fork out some money (don't know how much) for the supported workstation versions of Red Hat or SUSE.

One more thing I found out about Linux - some of the more exotic motherboard features may not be supported, and depending on what components the motherboard uses, some standard features might not be supported as well, like temp and fan speed monitoring on my Fedora box.

Hope this is of help (if you got through all my ramblings that is)