Dells tend to use custom hardware, and espescially on the ones with on-board graphics they omit the AGP slots. It's not that uncommon to see Dells and HP/Compaqs with 3xPCI slots and no AGP.
As for the replacement graphics card, forget about ATI - They don't make any PCI cards to my knowledge. (EDIT - Ignore that, there are ATI PCI Radeons available. I really should learn to read

Well, live and learn

)
nVidia do, but they are VERY hard to find, and actually cost a fair bit. You can't get high-end versions within a line on nVidia PCI, but the lower-speed ones should be fine. (e.g. you could get a Ti4200 but not a Ti4600).
Your best bet will be mail-order, espescially the more specialist places. I don't know about where you live, but finding gear like that in a shop in the UK is almost impossible

As for the speed, don't worry about it. AGP is VASTLY overrated - Running my Ti4200 at AGP 1x with Sideband Addressing and Fastwrites OFF runs at almost the same speed as AGP 4x with SBA and Fastwrites ON!!
The only place it makes difference is load times (Because the textures get transferred faster) and on games with really high poly counts, and even then the speed difference isn't as bad as you'd think. The FPS extremes fair a bit different, but it doesn't affect the feel as much as you'd think...
I'm not saying AGP is a white elephant - the performance boost is there - But if tweak your game so that you can fit all textures in VideoRAM and the T&L is caching trangle data (Which most modern cards do IIRC) then the amount of data being thrown into your card will low enough that PCI can cope and you will still get 30+ FPS.
So despite going PCI you're not loosing out as much as you might think. Heck, a PCI Voodoo 2 still slaughtered the AGP TNT's in terms of FPS

PCI-X looks promising but never underestimate corporations's ability to screw things up

But when it does come out, we'll have to essentially replace all our gear anyway so don't bother waiting.
Oh, and when it comes time for the next full-computer upgrade, build it yourself!!!

It is a VERY valuable learning experience
