This day in age if your not willing to upgrade your PSU, its a waste of time getting any decent hardware never mind a new GPU. What is the efficiency of your current PSU?
Depending on what games your are wanting to run there is a small choice to be made.
Are you wanting to take advantage of DX11?
If so you WILL HAVE to upgrade your PSU because its simply not powerful enough for the type of GPU your gunna need (Unless you got for the low end or mid range cards). ATI have lowered the power consumtion on the new 5xxx Series but i seriously doubt it will perform well (if at all).
If your not that bothered about DX11 and would like a card which takes advantage of DX10 instead then i suggest going for one of ATI's Radeon 4xxx series or Nvidia's 8xxx Series. Remeber, all your other components are sucking power out of your PSU also.
It's your choice at the end of the day mate, however, if your wanting to run the latest and greatest games aswell as have a good user experience your gunna have to upgrade your PSU. there not exactly expensive and if you get one of the last Gen cards you will have money left over for a new PSU.
I own both an ATI 4850 aswell as a Geforce 8800GT and the 4850 beats it hands down. Im upgrading to an ATI 5850/5870 (havnt decided yet) and the first thing im taking into account is the amount of power needed. Go ahead and buy a monster GPU if you want but dont be disapointed if you get a cycle of reboots or mad artifacts all over when playing games.
Just a bit of friendly advice. Hope it helps, Lemme know how you get on.