Well overall, AMD's "####+" rating will relate to equivalent P4 speed generally. IE, an Athlon XP 2000+ will be generally slightly faster than a P4 2 Gig. Some games particularly will favour the P4's (such as Quake 3) and some will favour the Athlons, (such as flight sims that use a LOT of FPU like AH). It would be fair to compare the 1.8 Gig P4 to the Athlon XP 1800+, they would be very close in performance.
I think the best bang for the buck in the Athlon line is right at the 1900+ mark right now. I'd go with that. It will be faster overall and in AH I'd bet than the P4 1.8 you priced before. The two systems compare pretty well othewise... they'd be quite close in performance, although an XP 2000+ would be faster than the 1.8 Gig P4.
As for the USB ports.... well yep I suppose it does have 2 less USB. This could be easily solved with a USB hub if you have more than 2 USB devices though.
As for the DVD, if you are not watching movies on it a DVD drive is a waste of money IMO. If you DO plan to watch DVD's on your computer, then be sure to get a hardware decoder instead for better performance. Other than movies, there is almost nothing coming on DVD yet for the computer. The only exception I know of is Microsoft's Developer and Technet subscriptions. Otherwise a DVD ROM drive is a waste of $ in my view. A regular old CD-ROM drive should be at least $50 cheaper than a DVD-ROM drive.