I run it on both.
I have 2 custom boxes.
asus a7n8x-e deluxe w/ ati aiw 9200 and a AMD Athlon-XP 3000+
<- I wasn't aware 6 months ago when i first began to use linux that it didn't have full support for everything when i built it. But that's only affecting the tuner and s-video in/out of my ATI All-in-wonder. everything else work great.
asus a7n8x-x mb w/ aopen m64 gfx card and a AMD Athlon-XP 1800+
<- this is my little brother's box. He thinks linux is ugly so he doesn't run linux but the motherboard is identical to mine, minus the sata contoller, gigabit ethernet, firewire, propetairy wifi card support, and has the realtek onboard sound (instead of sound storm). It also didn't come with the cords for the joystick and 3rd usb port (it is the usb for using the usb port on the front of a case, if your case has them anyways. my mb came with an adapter so i could use a pci slot and have two on the back. which was nice since the orignial case didn't have front usb ports.)
I also have 3 pre-built pcs and 2 laptops.
800Mhz p3 gateway
<- everything was detected fine. runs great.
500Mhz amd k6-2 compaq presario
<- linux seemed to have issue with the onboard video but some manual xorg.conf tweaking fixed it. xorg didn't want to go over 640*480, had to manually enter the amount of ram that the video card was using.
266Mhz P2 Dell Demension XPS D266
<- despite the slow proccessor it handled linux like a champ! It preformed between the compaq and gateway.
233Mhz p1 Dell Latitude CP
<- everything works but having only 64MB ram it can barely run a light weight graphical environment. command line was decent though. it's currently running win95 until i figure out something better.
1.8Ggz centrino gateway M275
<- everything but the tablet was detected at install. had to upgrade the wireless drivers since the ones that were with the distro had no wpa support. it was an intel 2200BG. Once i discovered that the tablet was on /dev/ttyS1 it was only a matter of adding it to the xorg.conf.
In my experience linux has supported prebuilt pcs very well. you might have to buy a hard modem since most modems are soft modems(aka winmodems). Winmodems are not mostly unsupported. Though in general, purchasing periphials with linux support can be a real pain.
Especially printers. I'd advise you to goto http://www.linuxprinting.org/
before considering to buy a printer. generally you should avoid all-in-one printers period, i've had pretty bad luck with these. lexmark as well. HP seems to have a wide variety of linux supported printers (http://www.linuxprinting.org/printer_list.cgi?make=HP
). From my understanding if a printer supports postscript you don't need a driver to use it but to get more than general support for that printer you need a PPD file for that printer and linuxprinting.org will probably have a PPD file for download.