I think the only problem you might have would be your power supply, it might need upgrading if it's 300 or in there.. as long as you have a decent PSU, you should be fine.
The only other problem is, when paired with a 1900+ CPU, the 6800GT will not fully be able to shine, if you do not plan on getting a higher end CPU fairly soon, then I'd get a 6600GT instead, as that will be a better match for your CPU. If you are planning on upgrading to a P4/A64 soon, then a 6800GT will be a great upgrade. But also keep in mind that PCIE is the new thing, and having an AGP card might also suck if you plan to buy a PCIE mobo. just a thought.