An Athlon 800 aint much to go by....nor is it going to give you more performance so to speak than your PIII. With that processor I can gaurantee it's a Via and it's going to be more of a problematic system than you would want to deal with. Win2k or WinXP is a better choice than 98 to be honest.
Once you know what type of mobo it is we can further help you on that
Opus

Whew, I have never had so much difficulty getting a computer set up. It's a pretty long, involved story...

Anyway, I'm getting a Duron now, 1.2 Ghz. It's a Gigabyte Mother board GA-7VKML. VIA chipset. It is not running yet. Keep your fingers crossed. I have gone through a lot so far and I have a feeling it might get worse. But, I could end up with a free upgrade for a few of my peripherals depending on what the store did to my system...(long story if anyone wants to hear it).

I really just need to know what OS to use (98, 2k or XP). I'm leaning toward XP. I also assume I'll have to work out details with the chipset stuff, I assume.

phat

By the way, I've been using a PII until now. (it even has an ISA slot) So I do expect some improvement in performance. Hell, the onboard video in the Gigabyte is probably faster than the PCI video card I have now.

Kinda sounds like that puter store is taking advantage of you still. They're telling or LETTING you buy a Duron processor??? That should throw up a red flag right there! I don't know what line of crap they're feeding you, but BEWARE!!!
Athlon XP 1600+ (1.4Gb)... $52.99/free shipping!
http://www.newegg.com/app/viewproduct.asp?description=19-103-303
Will work with that mobo ... no prob. Even though it has a VIA chipset, that aint a bad mobo.
Feel free to ask before you spend. We have no monetary interest attached to our help, so we won't steer you wrong. That puter store probably has dozens of those piece of shit Durons that they can't get rid of except to unsuspecting customers.
Do you plan to use this puter for DAW recording?

You misunderstood. The store is not giving me a "deal." My friend literally had the mother board "lying around." I'm getting the case chip and board for a good price (which is to say, pay me whenever you get around to it).

phat

Aaaah, I see. To be honest, you won't see much improvement with that 1.2G Duron over a 800 Athlon. Duron is AMD's cheapie line. For $52.99 you can have a processor that will make a very noticeable difference. Even if you don't have a steady job, you can work for a temp service for a day and make enough $ to cover it!
You still never said if you're using this puter for a DAW...