hydroxi :
in that case would i need a new psu if i was just switching from a gts 450 to a 7850 or 7870?
They'll run comfortably on a decent 450W PSU; the HD7850 on good 400W PSU. With a single PCIE power connector, the most power it could possibly pull according to specs is 150W; 75W for the slot plus 75W for the single 6-pin. That would be running flat out, not just playing games. With a HD7870, according to the PSU sizing calculator at
http://www.extreme.outervision.com/PSUEngine you would need 414W (minimum 364W). Subtract 20W if you go with the HD7850. Now that was based on my assumptions about # of drives, fans, and 10% capacitor aging. My assumptions could of course be off. Also, that was stock; if you overclock, you could easily add another 20W or more.