I doubt it. The 9800GT will be pulling as much as 125 watts all by itself. You'll probably want at least a 500 watt, maybe even a 600 watt. Given that the price difference is usually pretty minimal between them go for more wattage and err on the side of caution. Besides that, if you expect to do any more upgrades you'll have to allow for any future power needs. Getting more now will save you another PSU upgrade should you decide to add something else. A bigger one now is always cheaper than a smaller one now and a bigger one later.
A power supply is most efficient when run at 50% of its capacity. The second most efficient load is around 70%. A higher load works the power supply harder and dramatically shortens its operational lifespan. Your 300 watt power supply is probably being pushed to its limits. I wouldn't expect it to last you much longer.
And don't go cheap just to save a few bucks. Good brands give you a better product, one that will last you longer and protect your other hardware in the event of a catastrophic failure. I've seen plenty of motherboards fried by cheap power supplies failing. Antec, Codegen, Corsair, OCZ, Thermaltake, BFG, and Rosewill are some good brands.