Hello and welcome to the AT Forums. I must warn you; most (not all) folks on this particular forum are Hardware Nuts. Certifiable Nuts. Um, I'm about 50% Hardware Nut...but I have a small part of my brain that keeps my cash in my bank...small part, but still there. To a lot of folks here, an 800MHz CPU is "slow". Not to me, just kinda giving you some advance warning.
Oh yeah! Your question. Heck yes.
You already have a monitor, keyboard, mouse, modem (or NIC) and soundcard! That's about $300-$500 in your pocket already. Here are some approximate prices (I do check them a few times a week, so I'm in the ballpark). These are just examples. Others will point out countless more.
Abit KT7 Mobo $100
1 GHz (1,000 MHz!) TBird $117
Decent HSF (Heatsink/Fan) $20
Decent case w/AMD approved powersupply $80
128Mb PC133 memory (from Crucial of course)$50
GeForce MX vidcard $100
GRAND TOTAL: $467 + shipping
If your HD is more than a few years old, get a 40 gig Maxtor for about $140 or so...a worthwhile investment. So, for a really nice system that will game pretty good and surf/spreadsheet like nobody's business, you're looking at $400-$600 + about 5 hours of your time (If you've never put a system together before...if you have put one together before, you already know what's involved)
ps
You'd need an Operating System (OS aka Windows 98/ Windows Millenium etc)...if you don't have a Full Install disk already...well, add at least $100 to the total.
What I've listed will build you a pretty fast system at a reasonable price. If you shop carefully, you could get every cheaper. Good luck and welcome back!