Great computer
Your choice in DELL is great, and it has tons of horsepower (3.2 GHtz) and 512 MB Ram is very good too, but mind you.. unless you paid a big upgrade price (i'd wager you did not), your hard drive speed is probably 5400 rpm rather than 7000 (as is typical now for desktops). The 5400 rpm can be a problem for audio recording, I think. But let's suppose that is not a problem at all and move on to the issues you are facing next:
What I wrote earlier remains true (Firewire would be better recommended than the USB 1.1) which it's always better to go with greater speed and throughput, when you can.
Your new DELL will have USB 2.0 ports on it, but unless you get a USB device that was made explicitly for USB 2.0 (such as the Mackie Spike sound card), it will only go as fast as the USB 1.1 standard of a maximum of 10Mbs.
Firewire is 400Mbs.
USB 2.0 is 480Mbs.
PCI is like, I don't remember, but like 10x greater than USB 2.0
So.. your computer hardware is great (except for maybe the Hard drive's rotation speed and seek time). Now, you need to match up your recording software with a sound card. At this point, it's best to find out from the software manufacturer (before you buy), which sound cards they support. For example, you can check
www.cakewalk.com to see all the sound cards they have personally tested and recommend for Sonar (there's a bunch).
Personally, i think the prices on inexpensive USB and Firewire units are almost identical. A brand new USB Audiophile can be had for $190 including shipping (on Ebay) and so can
the Firewire Audiophile. Like no difference at all.
Another thing you may wish to consider is that the M-Audio products usually include some free lite versions of recording software that you may be able to use instead of buying something, and you are guaranteed that the free software (i.e. Ableton Live audio recording, and Reason Adapted) will work with the product you bought.