Need a little help pls

Yeah so I got before graphic card radeon 9000 and I played 1024 res low details. Now I bought a nice little gf6800 GT 128MB ASUS and my fps is sometimes 60 but most of the times in server where theres even small amounts of peoples in there I get 7-30 fps in high or med details.
Now I would like to know whats wrong at my pc,,,what I should buy to get it run smoothly. I know I should buy lots of thing but what is most critical thing?

System Specs:
AMD Athon xp 2000+
gf6800 GT 128MB
756MB RAM 300MHz
350W powersupply
MSI K7N2 Delta Motherboard
Creative soundcard...cant remember the name of it
1xCD 52x32x52x
1xDVD hmm not sure was it at speed 20x
Case temperature 29 Celcius,,,Processor temperature 47-52 Celcius


I think its because of the powersupply but Im not sure, so I ask your help.

yes ps and the processor.

Thank you for quick reply :)
Yeah so new processor also as quick as possible

New Proc, New MOBO, new RAM...those are the most critical...
if you can afford an SATA drive...get one
you got a good vid card

An HDD will have absolutely no bearing on FPS.

756MB RAM 300MHz

there's no ram that runs at 300MHz, nor can you have 756MB.

as for upgrading, get a new processor.

:P okey 333Mhz and its 1x500MB slot and 1x256MB slot
...huoh

sandpaper invar?

:P okey 333Mhz and its 1x500MB slot and 1x256MB slot
...huoh

It's 512.