I just switched to Windows Vista recently and my main concern was whether or not I could install all the programs I used to use on Windows XP. So far I have not had any problems. Programs I use:
7-zip (for .zip and .rar files)
CCCP codec pack (to watch divx/xvid movies)
uTorrent (to download BT files)
Nero 7 (burner)
PowerISO (virtualize CDs)
Office 2007 (well, everybody needs this....)
BSPlayer (for watching videos)
AIM6 ad-hack (to remove AIM ads, etc.)
Skype
Symantec Anti-Virus
Firefox
PHP Designer
Wampserver
etc. etc.
All of these programs are Vista ready. What else do you need?
Vista is the latest OS and eveybody is moving quickly and porting software to Vista. It's inevitable. If you buy XP now you are seriously losing the advantage of getting Vista through Dell because Dell heavily subsidizes the OS for you. If you decide a year from now you want Vista, you will have to pay full price for Vista which will come at a huge cost. If you get your PC built with Vista now, you will receive the benefit of getting it bundled by the manufacturer (in this case, Dell).