Will Enermax Liberty 620W be enough for ATI HD4870X2

Hi all , I will be getting Gigabyte Radeon 4870X2 soon , just wondering will my Enermax Liberty 620W be enough for it ? I am currently running 880GTX just fine , and I dont have any other heavy power load components running

Recently I just realised that my 3Dmark 06 result is all over the show , the frame rate is really laggy when it is suppose to be reasonable fast , sometimes I would restart or shut down and turn the computer back on , it would fix the problem , but sometimes it wont.

the first scene of 3Dmark 06 , normally I would get around 4200 frames in total, And now its half around 2000 frames in total , and I remember shutting down and rebooting the computer would fix it , now it wont.~ but the thing is , it doesnt affect games like Warhammer online or fallout 3. I am playing it on 2560x1600 all maxed , the fps doesnt feel laggy. but I remember last time I was playing fallout 3 the fps went really bad , and it restarted with a blue screen. After the crash it went all back to normal performance

Any idea guys ?

my current spec is

Core 2 Duo E6300@ 2.66ghz
Win XP 32bit Professional SP3
ASUS P5B Delux Wi/Fi (BIOS 1004)
Balastisk Tracer 1066 Dual channel
Enermax Liberty 620W
EvGA 8800GTX ( 175.19 driver) - No overclock have been done to it