Make sure it's an athlon and not a sempron. I've seen companies call them AMD 2400 or AMD 2800, and people think wow it's an athlon great, but it turns out to be a sempron which is the equivalent to a lower end celeron
Back in Pentium 3 days they were mostly comparable (though AMD had a slight lead), then Intel realized that people didn't understand CPU performance, and that higher megahertz numbers meant people thought they were more powerful.
So, they took the marketing opportunity and produced a chip (the P4) that was less powerful than a P3 at the same clockrate, but could be pushed to huge speeds. They had Itanium for high-end processing.
So, AMD started marking their chips with the pseudo-MHz ratings -- my Athlon 1800 doesn't run at 1800 MHz; instead it gives comparable performance to a P3 at 1.8 GHz.
Anyway, while Intel was milking the money from easy marketing, AMD was making better chips, like those that can run 32 and 64 bit code with no penalty (the Itanium sucked with 32 bit legacy code), and designs that offered a memory layout that allowed multiple processor builds to scale much better.
Anyway, AMD offers better chips right now, but the hyperthreaded Xeons you can get from Dell ain't bad either. Building a box? AMD. Buying? Not a whole lot of choice, but Xeons aren't bad by any means. Give it 18 months and Intel may jump ahead again. Or not.
You're buying for now though. Just buy a good box and don't sweat the details.
AMD has great chips, but because of the motherboard
My only AMD PC right now had a problem with its network. When I set up the cable modem, I just couldn't get the cable modem to work from my AMD PC, until I plug in my ThinkPad and everything worked from there.
The strange thing is, it works perfectly when connected to the router.
At my work, I also had problems with AMD based computers for all kinds of problems. As far as I know, Microsoft had some AMD specific patches.
I have to say AMD is really fast, but you have to consider the reliability of the whole system - including motherboard, drive/RAID controller, drives, memory... AMD had the distadvantage of relying on third party for motherboards, and it shows. I would be hesitate to use AMD for mission critical server.