I've been looking into this too... I have a few leads.
One, Leadtek seems to have very good luck in the 4700.
Two, heat is a MAJOR issue! The 6800's generate LESS heat than the 6600GT, which is one big reason why so many 6600GT's and Radeon x700's are having big troubles.
Three, go for good venting! There's some two-slot cards that vent all that hot air out of the case - that would be an excellent choice. Another option is to get the $20-25 3rd-party cooler that does the same thing- mount onto the side of the graphics card and vent the hot air out. Apparently the 4700 just doesn't have good airflow....
Four, there may be some tricks to loading the drivers. I'll try to grab them off the Dell support site for you... shouldn't be a major issue though.
A nice fella' recommended a Radeon X800 with that 3rd-party fan/vent. Came to $300 in total.... not bad for a super-high-end solution!

On the budget side... there's lots of $99 Radeon X600's to be had, and if you have trouble keeping it cool, you can add the $20 vent to it. I think for a hundred bucks, the X600 is tough to beat.
But I'm not the expert here... I'm still looking... but we're in the same boat and I'm just posting what I've found so far.
And man, did I ever get my 4700 for a STEAL of a deal!!

DVD-burner, 160gig, 17"LCD and the whole thing was only about $500US with free shipping!