I just bought an HP d5000t which has a 460 max power psu. it also says on the psu that it has +12va+16, +12vb+15, +12vc+8 something like that. i was wondering if it is able to run ati 4850 max results without having a problem.

You should have no problem. Just make sure to remove the nVidia drivers first.

You might have a problem. The +12Vb (more commonly called +12V2 ) powers the CPU. The +12Va (+12V1) powers pretty much everything else. The +12Vc (+12V3) likely powers a 6-pin graphics power cable. I see at least two potential issues:
1) If the PS could supply +12V1,2,3 max currents all at the same time, that would be 16A + 15A + 8A @12V = 39A @12V = 468W just for the +12V part of the power supply. Since the PS is rated 460W total including everything, all the +12V maximums cannot be supplied at the same time. Thus, your power supply may be too wimpy for the high-power 4850.
2) Assuming the +12V3 is for the graphics power connector and can supply the full 8A, that's 96W, which may not be enough for the high-power 4850.

Bottom line: try it, but don't be surprised if you need a higher-power supply to run it.

If you consider the requirements of the 6-pin PCIe connector, namely 75w, plus the motherboard pass out, it will be enough. Plus, the 9800GT 1GB uses about the same amount of power (maybe a touch less), and is an available option when customizing the build at the HP website.

i looked at the psu again and it says 12Va+12Vb+12Vc cannot exceed 365 or 380 Watts. something like that.

I have a d5000t and just added a 4850...no power problems evident. However, the digital cable tuner now produces a decoder error - which seems to be DRM related. Still working on that one.