That 4850 is definitely better than the 9800GT.
My 8800GTS (512MB G92 version) outperforms the 9800GT. All the 9800GT is, is a relabeled 8800GT. It's the same chipset, same performance. My card outperforms the 9800GT, and the 4850 outperforms my 8800GTS.
Plus, if you want to go Crossfire, all you need is a second card later.
If you go nVidia now, you'll have to buy 2 cards later to go Crossfire.
Make darn sure your "Decent" PSU is a good brand. Also make sure it has the appropriate amps on the 12V rails to support the 4850 (let alone dual cards).