well i want to help you the best i can so i had the dude who's upgrade i did send me some quick pics. i circled important parts as necessary. these are really BIG so i'm linking them. so this is YOUR DELL exactly. save these for future reference
the first one shows two circled areas to the right
pay attention to the red and orange FLAT cables in the blue circle. ignore the others. those are the cables i'm talking about that are in the way of the video card. so they must be routed around
circled in red in this next pic shows the cards length. as you can see ANY length card will fit so long as those SATA cables are long enough to go around it
ok i wanted to make sure of the dual slot issue specifically. and you may run into some trouble here. which means it may be trouble for the card you want. look at the red line in this next pic. its drawn on top of a pci card. right above the pci card you can see a flat silver box to the left. that is another pcieX1 card taking up a slot. you just can't tell from the pic. and this is where YOUR dual slot card would find room if YOUR slot is empty. above that you can see the single slot 8800GT
now if you look to the right of the photo where the red line ends you will see some flat blue and red cables. THIS is the problem area
in the next photo clrcled in yellow, that is the BOTTOM of the installed vid card. or the top, however you prefer to see it. its the heatsink and cover, since vid cards hang upside down. anyway, you can see its reflecting stuff from the flash, so that is the 8800GT. the encircled blue is where the dual slot cooler would fit....encircled in red....is the trouble
the red and orange cables you see in the circle are SATA cables from the dvd drives. the blue is the HD cable. as you can see, a dual slot cooler would impose on these cables somewhat. now, the card doesn't seat all the way down on the mobo, though. i thought it seated lower, but there is space between the card and the mobo
so, in my opinion, the only way to get that 8800 Ultra in this unit (or any other dual slot card with a large heatsink) would be to replace these cables with longer ones, plug them in BEFORE you install the card, and install the card on top of them, as you position them to stay out of the way. so the SATA cables would have to be long enough to flex, bend, and comply so as no to interfere with the dual slot cover and vice versa. in this instance they would run UNDER the card rather then around
however, look at the pic again. this unit has two dvd drives, and it looks like he has taken out an HD since i worked on it. so it has just one HD. look at the pic again. you see the red and blue cables? one is an HD and one is a dvd drive. if you have just one HD and one dvd drive you can run them both from these plugs on the mobo, leaving the two up top empty. this way yes, the heatsink of the 8800 Ultra
should fit without doubt. because nothing would be in the way. just run the SATA cables around the card like normal. and as said, the pics don't show that the vid card clears these plugs on the mobo. its raised above them. if just barely
so imo a single slot solution is most convenient. but a dual slot is not out of the question with some effort. also, not all dual slot cards have full covers/heatsinks. as you know some just have large dual slot fans with no cover. so those would also be an option. but anything like the 8800 Ultra will offer you some extra challenges
this next one is just FYI
see the wire circled in orange. thats just from that case fan. i'd recommend you do this. case fans are cheap, try to get a quiet one because the Dell's is quiet. so if you want to keep your computer quiet get something you can live with. however the Dell fan is quiet because its hooked to the mobo and doesn't move enough air imo. its maximum rpm is something like 1200. the one on this case moves at 2400rpm constant. so you just take one out and put another in and plug the other right into the psu, not the mobo. so make sure it has a molex connector or an adapter. so far as memory cooling or cpu cooling, nah, you're ok. but if you want to make changes....one thing at a time. but no. especially on the memory side
oh, and that stuff circled in blue is the HD and the area below is where another would go in your case. which may or may not be hindered to fit by a dual slot cooler. but i would doubt it
hope the pics helped. anymore questions feel free to ask