The Radeon HD 2900 XT will overclock to 840 MHz on air and if you haven't seen that story you can read it here. The 840 MHz core speed of the Radeon HD 2900 XT card is possible with the reference cooler.
Even if you don't have the 2x4 pin necessary for overclocking you will be able to overclock. The plain two 2x3 pin power connectors will do the job but the Catalyst control center won't recognise that the card has the Overdrive capability.
All you really need is the GPU Overclocking tool that we mentioned and linked to here and you can overclock even with two 2x3 PCIe power connectors. So you won't be needing a new PSU.