I can get the powersupply later right?,It wont affect the install? i have 300watt right now
IF the Powersupply does not have enough power to run the new video card then the system will either be unstable (if it is close but not enough under heavy usage) or not run at all (if it is way underpowered) - Worst case would be that it has enough to run for awhile but winds up burning out while running a game (as the power spike from it dying could also damage the new video card or the MOBO or CPU or other parts of the hardware ! - So you'll want to upgrade the powersupply either before the new video card or at the same time !!
The power specifications for the 9800GT call for a minimum of a 400W powersupply so trying to run it on a 300W system would be asking for problems.