DeadSeaSquirrels, high-end x86 servers come with Xeons, that's just the way the market's currently played out. Xeons are good chips, but totally overpriced relative to their performance - but since you can't buy higher-end system hardware for any other CPU, you're stuck paying. In general, on the high-end server kinds of boxes, everything has been racheted up a notch nicer and several notches more expensive, and that's just the way it is.
For a SMALL business, you can build boxes that will work nearly as well for a small fraction of the price. A quality Athlon XP system will just win on price/performance as long as you don't need the highest end performance or reliability. Remember: to get a little more performance a little more reliability, you pay a LOT more to jump to the high-end hardware.
Intel has solidly targeted the medium to large enterprise server folks, the people who have several or many servers and need performance and reliability more than they need cost savings. Remember, man-time is expensive, and so down-time and maintenance is a hidden cost; often buying a more expensive piece of hardware is cheaper than not doing so and paying it in man-time. When you get to the large enterprise scale, this can start to matter. (of course, large enteprises are also institutionally stupid and cost optimize in the wrong ways, but anyway)
AMD has to date been unsuccessful in the medium to large enterprise / high-end server market. The image perception of a "value brand" as compared to the conservative choice hurts them, as is the image perception of being a "cloner" or a "trailer" compared to Intel. And also, frankly, Intel is huge. Intel can devote the internal R&D and marketing resources to just the server market that AMD has total. No big surprise that Intel is more successful at it.
AMD is once again pushing with Opteron, trying to get into the high-end server space. It appears that they're getting design wins, which is a major achievement. Intel just plain screwed up with Itanic, and their recent x86-64 switch after years of denial has cost them - they're spinning furiously trying to prevent people from figuring out that AMD one-upped them. The problem is that Intel still has a very powerful channel and marketing relationships, and so while AMD got a lot of the design wins, the server boxes that move in volume might still be the Intel ones. Time will tell.
There are some really interesting "server" instrumentation, management, and reliability features that are in modern hardware, though the software support is currently behind. This is the kind of thing you gain by buying a higher-end "server" system - for example, IPMI. When you have a few racks filled with Dell PE2650s, you really want those tools, they make your life easier. And all this stuff, you just can't get it on an AMD right now, mostly as a consequence of AMD not really being successful in the server market.
I wish AMD luck, though - WE, the users, desperately need a credible second source in the higher-end server space to keep Intel honest and to bring down the prices.
But anyway, this is getting way off topic for networking.
Yes, I use AMD Athlon processors for servers I've custom built, and they work great. I can't buy a big "server" from any major vendor using Athlon, though, so it ends up being a custom-only choice for me. (and I'm trying not to build PCs more than I have to... my time is expensive, while Dell's and Compaq's isn't...)