are you sure it has a pci-e slot? all of the dell specs list the slot as optional. Also which model is it? The different form factors have different power supplies. The biggest psu in any of them is 305w... which you have to run your entire system on. That doesnt leave a lot of room for upgraded graphics. The amd 6850 which would give moderate performance has a power draw of 121w (according to guru3d) and that wouldnt even fit in your system ... That is of course assuming you have the highest end CPU they offer in a pentium4 w/800mhz fsb... those are 105w processors IIRC.
Rough estimate on your power draw right now... 215w. Thats assuming you have a 3.2ghz northwood p4 for a cpu.
Your system is honestly terribly outdated at this point. Your best bet is to buy a new computer... or build your own. If you build your own you can customize everything including the power supply. Then you can put in whatever graphics card you want as long as you put in a big enough psu. Then when you want to upgrade later you wont be hamstrung by Dell's custom power supplies and small form factor cases.
No doubt about it, you really can not upgrade that system without BOTH a power supply and a new video card (GPU)......
Even if you have the 305watt supply, it certainly no Longer outputs full power due to it's age (capacitors just aren't what they used to be...). It's likely more like 250watts or less which won't allow the addition of any new GPU.
I would double check to assure you have a full PCI-E expansion slot on that motherboard. List detailed model number of the MB and we can help confirm what you have
Here's a list of power supply size requirement for GPU's: (no cards can be supported with less than 300watts..)
oh also your pci-e slot (if you have one) is 1.0a which means you will be possibly screwed to get a 2.0 card since several 1.0a boards arent compatible with 2.0 cards... even if you could get a card to work with that power supply.