Only you can answer that question. Do you need the extra cooling? What are your future upgrade plans? Is the case really nicer looking? For me the answer was "no" on all counts, but I'm sure XPS owners would disagree!

I have an XPS and I would agree with that. The extra cooling isn't really necessary (and some would say it adds a whir to the room although I haven't noticed it with my XPS)

As for its ability to be upgraded, it does have more room and opens up very easily. But at the same time, I don't really have any computer expertise to be adding things and more likely, I'll end up using the system as is until it gets old and just get a whole new one.

Lastly, I found the XPS to be almost too large and cumbersome compared to the 8400 (my roommate has an 8400). It doesn't really look that much nicer.

What I ended up doing was waiting until Dell had a good coupon deal and got my XPS for $1700 with a 19" flat panel monitor. I thought that was a pretty good deal and fairly close to 8400 prices!!

The XPS PSU has higher power output for upgrades and better case ventilation for cooling.  Consider this especially if your looking at running a top of the line Ultra video card in the future.  I have both an XPS and an 8400 configured nearly the same, with the exception of the processor 3.6 vs  3.4 and video card.  The 8400 works very well, though still not as well as my XPS. When I purchased my XPS DELL was making room for the new Gen 4 models so I got a really great deal on mine.  For me the extra $150.00 was worth it. :smileyhappy:

Yes! if you can cope with the noise? But then again if you have no air con you will when you switch

The XPS, if running properly, is very quiet regardless of all of the fans that it has.  There have been a few, mine included, that had faulty heatsinks which caused the CPU fan's to run at full speed (it literally sounded like a hair dryer on high setting).  But then again some 8400's are suffering from similar problems with fan noise also. Once the heatsink was replaced it was unbelievable how quiet this computer had become.  I have both an XPS Gen3 and an 8400 and the 8400 is very quiet while the XPS is just slightly louder than that (and I mean SLIGHTLY).  To me the difference is negligible.

If someone compares an XPS's cooling fan noise with an air conditioner then I would venture to guess that their XPS  (Gen 3 or 4) is not operating correctly.