I would not do a PCI video card myself. Check with Dell to see if you have a AGP slot on your motherboard. If not, then you will have to do a PCI video card. Be advised that AGP and PCI video cards look similar, but they are NOT interchangable! AGP slots are usually Brown in color, and PCI slots are usually White in color. The AGP slot is also offset from the PCI slots too.
Audio applications do not use 3d rendering at all! So, forget about getting high end "gaming cards"! They will be useless for your DAW. Any ol' modest 64mb video card will work just fine. You could get away with a 32mb card, but I don't think you can buy them with that little memory anymore! LOL But overall, don't worry too much about specs on the card. Be more worried about the chipset used! NVidia GeForce chipsets have tranditionally worked better in DAW's from my experience (and many other peoples experience). I have not had as good of luck with ATI Radeon AGP cards!
Radeon AGP cards don't seem to play that well with devices on the PCI buss for some reason. Some claim it is their drivers are not that well written, others claim the chipset is just not that good. Either way, just from experience, I would avoid them. NVidia GeForce chipsets seem to work great though!
More RAM is not going to gain you much of a performance boost in your DAW UNLESS you are running VSTi or DXi instruments. Don't confuse those with VST and/or DX plugin's. VSTi and DXi instruments need to store sounds in RAM to work, so if you are using HUGE sound libraries for these, then more RAM will help them work much better.
Otherwise, CPU is your friend in a DAW. The more the merrier! All your plugin's, fade's, crossfades, volume changes, etc...all use CPU cycles to perform. So yes, having a dedicated video card will free up some CPU for sure, and will help things along.
I agree that a dual head card is the way to go! I have a MSI GeForce FX5200-128mb DDR video card that costs about $35!!! I run two 19" monitors on this card, and it is wonderful!