That Dell is faster at stock speeds. The 2.8Ghz P4 is faster than an Athlon XP with the same "speed rating" (ie 2800+), so it's obviously going to be faster than a 2500+.
If it were overclocked enough the AMD system could overtake the Intel, but it would have to be pushed well past 2800+ speeds to equal the Dell.
And I'm with Hellbender. Dell isn't half bad at all. I just got my first pre-built system in 7 years from them, and it's pretty well built and uses good components (I was surprised mine even used a Seagate hard drive). They're not nearly as proprietary as HP/Compaq, etc., and the designs are pretty non-gimicky.
EDIT: I didn't catch that you've already got it overclocked to 3200+ speeds. From the Doom 3 benchmarks:
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2149&p=7
it looks like a 2.8Ghz Prescott is just slightly faster than a 3200+ Athlon XP. Personally, if they're about the same, I'd rather have the system that's running at stock speeds, than the overclocked one.