Macs aren't impervious, I've seen OS X lock up a few times. Personally, I like the cheap PC hardware with Linux. Does everything I need/want and then some.
Of course, if you insist on a Mac, you can do what I did with a G3 based iMac. It is a combination of my mail/web/dns server and I use it for remote X sessions wiht my old POS laptop. Running Linux of course
Linux imac 2.4.25-powerpc #1 mer avr 14 15:38:38 CEST 2004 ppc GNU/Linux
22:10:34 up 26 days, 8:38, 0 users, load average: 1.10, 0.93, 0.52