Well, there's really only two things to consider and on average they scale (or we can for most purposes, pretend that they scale) linearly.
There's clock speed. Some have called this "pep" which works for me.
And then there's bandwidth. Number_of_Cores X Clock_Speed = bandwidth.
That's it. We can benchmark particulars but generally speaking, that's all there is. Pick the balance between the two (plus price) which you think you want and go for it. I mean if you're set on buying a new system.
Anything else is just an order of complexity added in that probably won't matter much if at all, in practice.
Another consideration is to get a $200 used system and run it as your server. And trust me a $200 used system these days
ROCKS! as a server!
EDIT: something like this:
Price $250
Clock 3.8GHz with HT
3GB Ram
73GB (SCSI/15000rpm)
COMBO Optical drive
Quadro FX3400
Windows XP Pro
KB/Mouse
etc.
http://page19.auctions.yahoo.co.jp/jp/auction/x68836645
Sorry for the Japanese but that's where I live. Anyway systems like this are all over the place in almost every country - in great abundance. They make great personal servers! I ran one off of a DEC Alpha 233MHz running Digital Unix up until just a little while ago.