As others have said, it really depends on your hardware...
If you choose the right motherboard -Thigns get super easy.. Right video card- Even easier. Those are pretty much the only 2 key parts.
Gigabyte boards have the most compatibility.
Here is my setup (a bit out dated, but it gets the job done)
Gigabyte EP45-uD3P motherboard
Intel Quad Q6600 2.4ghz
4GB of DDR2
Nvidia 8800 GTS- 640mb.
With this set up I installed 10.6 with zero issues. I only had to install 1 KEXT and that is optional (It just made bonjour work with my network card). Install was quick and painless- about as easy as my macbook. (There is also a Life hacker article on this exact setup).
I have also been able to use the built in software updated with zero issues. I am currently on 10.6.3 - use the computer EVERY day. NO kernel panics, NO slow downs, NO problems. It is as stable as my macbook.
Although, the future is unknown. The next update could kill it (or at least just cause some minor issues that need fixing), and that is the risk with these. It is usually best to wait on updates and see what others say first.
This all being said, my next desktop is probably going to be a Mac Pro.. But that won't be a few years from now as this computer is working great for what I need. I don't want a macpro because I hate my hack, but I feel that it is the next step to go.. Plus any thing I build will cost around the same as a mac pro for about the same specs (Will be using server parts just like the mac pro) so I figure I would just save my self the trouble
