AFAIK Linux supports everything you mentioned in your post. You shouldn't have any problems with memory, processor, etc.
Dell can actually sell you a computer loaded with Red Hat instead of Windows. Doing this would gurantee that the hardware on the PC would be supported by Linux. However,y ou can't order it right out of the wbsite. You have to call them and ask for a PC loaded with Linux.
I have succesfully loaded Linux on multiple Dell computers, without any trouble whatsoever. They are very stable machines. The motherboards are based on Intel motherboards, so you won't find any AMD stuff there. Most of their machines have integrated 3C920 network cards, which have pretty good support on linux. Some have integrated audio, which uses the i810 modules.
My computer at home is a Dimension 8100 and runs Windows XP/Slackware great. Everything on it is supported. It can go up to 2gb of ram (RDRAM, currently using 512mb. I think they changed to DDR on their newer models.) It supports a P4 processor up to 1.8ghz, up to 2.4 ghz with a special non-Dell kit.
My work laptop is a Dell Latitude C840, and again, everything is supported by Linux.
They have excellent support, too.
Now the bad part is that you can probably build a computer with the same specs for about 60% of the price, but you have to provide your own support and pursue warranties with stores and manufacturers.
For hard drives, I'd suggest them in this order : Seagate, Maxtor and Western Digital. Hard drive's life expectancy depends on use and the conditions you submit it to (heat, dust, etc.) You can probably get at least 3-5 year's life expectancy for a hard drive undder regular conditions.
For quality memory, buy from Crucial. RD-RAM is the faster, but most expensive (Crucial does not carry it.) DDR ram is similar to RDRAM but less expensive. SDRAM PC-133, then PC100, then PC66 in that order of speed. Depends on your motherboard what ram you should buy, though.
Altec Lansing speakers have a good price/performance ratio.
Soundblaster Live is well supported on Linux.
Nvidia cards are easy to set up and well supported (though non-GPL drivers.)
Ethernet cards tend to outlive a computer, as far as I know.
If you post in the Flea Market topic, I am sure someone can cheaply mail you some bay covers.