Ok, to determine prebuilt vs DIY:
What OS are you running? Would you like to move to XP if you're not already there?
Is noise a big concern? (Can build yourself a quiet one, takes more attention)
Do you expect your needs to change significantly in the next few years?
Is size a consideration? Full tower or shoebox size no difference to you?
Your time, do you enjoy working on your computer or would it be a chore?
Seems you get a computer working, use it for a good while. So whether a prebuilt has 3 or 7 PCI slots, AGP 8x or 72x, etc, doesn't sound critical. By the time you run into an expansion need that a prebuilt's motherboard doesn't support you'll be ready for a new system.
Most people don't include software when they price a prebuilt vs homebuilt. So they get them within dollars of each other and then declare victory because their pile of unassembled parts could potentially run Quake faster. If there was an OS to start the machine with, a mouse to click the Quake icon...you get the point.
Dell I got my mother is very quiet. Only 2 fans in the entire thing, a standard 80mm in the power supply and an 80mm just below it pulling air through the case and over the CPU via a duct. The only noise that comes from it is from the hard drive and even then only during seeks can you hear it. Very nice for an small home office environment like she has. Also, the P4 2ghz she has puts out almost no heat when sitting idle. (2k)XP+iNTEL=cool idling. AMD never has gotten their HALT commands as good so my 2700+ pours out warmth all summer.
Along the same lines with the noise/reliability - video card. If you get a Dell you'll likely get a GeForce 4MX or Intel Extreme Integrated. If you don't game much or at all these would both work, though I'd prefer the GF4 solution over integrated. It's better overall and no downside especially since the OEM GF4 won't have a fan. If you build a system yourself this is something to keep in mind when picking a card. Fans are nice in that they let gamers have faster cards, but meanwhile they're making noise and collecting dust every second a computer is on, gaming or not.
It'd be nice to have dual 8ghz Xeons with an ATI 99000Pro and 4 gigs of ram, but if you don't need it and wouldn't use it why pay for it? If I were you I'd watch Hot Deals, or for a quicker to scan option check
Ben's every other day or so for a Dell deal and go that route. Figure up how much it'd cost to build one so you have that reference, using the parts you actually need and the software you'd get and use from a prebuilt, then compare as deals come along. Doesn't sound like you're in a crisis with your needs and your current computer so no need for rash purchases.