I have a Dimension 8200.
Overall, the machine runs great and I've never had a problem with it, but it is getting a little long in the tooth.
Now for the negatives with upgrading:
It uses RDRAM, which is expensive. So upgrading the memory will be more expensive than with other machines. You can check out New Egg for RDRAM prices. The machine uses PC800 at either 45ns or 40ns. (Depending on whether its 400MHz FSB or 533 MHz FSB).
As for upgrading the video card, you should keep in mind that the 8200 typically had a 250 Watt power supply. So you might need to also upgrade the power supply. Also, it uses a somewhat non-standard power supply (although others have said that its a standard ATX power supply that you just need to remove some metal from to make it fit).
Another note, is someone told me that the space for the AGP card is small, so if you are looking at a thick video card, you might have an issue there. I currently have an NVIDIA GEFORCE4 MX 420 card (64Mb) that came with the machine that does everything I need. (Although I've been considering upgrading to a GEFORCE 6200 or 6600, which is how I learned about the power supply issues).
Also, mine has USB 1.0 ports (not USB 2.0) and no firewire, so you might want to consider upgrading those.
All that being said, the machine has been great for me and has worked flawlessly and I am also looking at upgrading it.
One final note, there is a guy on the Dell Forums that has a website devoted to upgrading Dell 8200's. He goes (or the website) by some Spongebob Square pants name.
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