Hey, I've been playing DD for quite some time. I also admit I don't probably have the best PC in the world, it's quite old (3 years), but I think it's still pretty decent. I can still run most games on high settings at least. (Like Skyrim on the highest with low shadows). I have turned on VSync, so the FPS is capped at 60. Tavern, no problems whatsoever. Normal levels with not so huge waves, still doesn't really drop below 50 at all. However, when I launch the Eternia Maps, the FPS is really most of the time 59-60, but when the waves come in, it tends to go to about 20, which is really still playable, but for me kinda annoying.
I have Pentium 4 dual core 1.8GHz OC'd to 2.4
ATI Radeon HD 4870 1GB
4GB RAM, however, XP 32bit sees only 3, because with VRAM it can be only maxed to 4.

I also have some spare Core 2 Duo laying in the house, and with help of my dad, I could get it OC'd from 2.4GHz to about 3.2 or 3.6 I think. Would that help out significantly?
I have tried tweaking the UDKEngine.ini file, and disabling some features, the one which could probably drop my FPS most could be the dynamic decals, but it also turns off the tower range radius things.
Can you help me out?