Ati radeon 4850 problems

Hello,I have an e-machines with 2 gigs of ram ati radeon 4850 and a 3.4 ghz processer now lately when I've been playing games that are more graphic intensive after some time it ethier freezes up or goes into power saving mode now i've read this can ethier be my motherboard battery being dead or maybe my 450 watt power supply isn't enough for my card help would be appreciated