Yeah, I would go with a 4850 because its more of a high-range card.
Although it is a step up to get a 4830, the money you spend may not justify the slight increase in power. Both the 8600GT and the 4830 are mid/lower range budget cards.
Also, don't worry about your PSU. It will run it fine (assuming it doesn't do what slmclarengt says). Personally, thermaltake stuff has been alright for me, but idk.
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Sony PCV-RX755 (old but still runs great!)
MOBO: ASUS OEM board
CPU: 2 GHz Socket 478 Pentium 4 Northwood
RAM: 2 GB Kingston ValueRAM
GFX: EVGA Geforce 6200 AGP
HDD: Samsung Spinpoint 100 GB ATA-100