As DougC may or may not have been hinting at, be warned that Dell help support has become very hit or miss since it has been ‘outsourced’ (or ‘best shored’ or ‘shipped to India’). For every good anecdote I read anymore, I read at least three horiffic negative ones.
If you are technically savvy, it should not be a problem, but at least wanted to make you aware.
“Attempting to upgrade a Dell is a risky operation.”
Not any more. Yesterday I pushed two buttons on my dell 4550 case, the case came open, I pushed two connectors, the cd drive came out, I put some new rails on my dvd writer, slipped it in & connected the cables. Took about 4 minutes.
Depending on how much you know about PCs, you may want to check out the “Configurator” at www.spartantech.com - You can choose every individual component yourself (Motherboard, CPU, heat sink, etc). They’ll assemble it for you and send it out.
I’m pretty sure that quote from gotpasswords was in reference to replacing the motherboard and CPU, as the OP seemed dissatisfied with her 400 MHz Pentium II. While the drive bays in most Dells will accept standard size disk drives, some Dell mini-tower cases might not accept a motherboard in the standard ATX form factor. If the motherboard on the OP’s Dell will not support a much faster CPU, then a significant CPU upgrade will necessitate a motherboard upgrade at the same time, and possibly the purchase of new RAM, if the old motherboard used a type of memory not supported by any modern motherboard. Completing the vicious circle, if no motherboard currently on the market will work with the OP’s case or her power supply, then the case might as well be replaced also, which brings us back to the drastic options suggested already:
[ul][li]buy a Mac;[/li][li]build your own computer from parts, or have a computer shop do this for you;[/li]buy a used computer on auction sites.[/ul]
I’m for upgrade the RAM to the maximum – as long as it’s not 128 – for dimes on the MB.* I run several machines under 400Mhz, with Windows 98 (SE only), NT, 2000 and XP on a 433.
If you can crank up the memory, get inquisitive about a processor upgrade for this motherboard.
If you’re low on space, buy a cheapie external USB hard drive. Rubberized case, portable, good for backup too.
Finally, get cozy with Linux. Linux will make this machine do incredible things…
Save up your money and wait past Christmas, then buy TWO new machines.
*With mail-in rebate, maybe even free these days.
“some Dell mini-tower cases might not accept a motherboard in the standard ATX form factor.”
The operating system software that comes with mine uses a bios lock activation too, so you’d have to probably get a new OS if you get a new board, but I haven’t tried using the OS disk I got with it on another computer.
Check out Crucial.com . They have a great site that allows you to input your computer make/model or your motherboard make/model and it will tell you exactly what kind of RAM your system takes.
As for changing it, it’s very simple as others have said. After you have purchased the new sticks of RAM you’ll easily be able to locate the existing stuff. Just pop out the old and put in the new (or sometimes you can mix -and-match. Crucial’s site should be able to fill you in on all the specifics).
Before going the Mac route, experience some truth in advertising. 
(It’s only a joke, some of my best friends are Mac users)