If the machine is a pre-build a la DELL / COMPAQ etc, then check with them before you go buying anything .. and same goes for the PSU .. is it a 300W TOTAL COMBINED output unit, or does it have 300W @ 12v ? Is it a cheap generic thing or a good brand ?
Either way, list what make / model with a link to the manufacturer's spec sheet for it or a CLEAR photo of the WHOLE label on the unit's side and we'll be able to tell you what card(s) might run off it .. ( would be a good idea to list your system specs too, as that's important to know too for giving advice on VGA upgrades ) ..
AM3 965BE C3
/ 4GB Corsair XMS3 2000Mhz / GigaByte 990FXA-UD5 / Corsair H70 & 2 CM Excaliburs
/ x2 HD3870 ATOMIC Edition / Corsair 90GB Force 3 SSD / Momentus XT 250Gb Hybrid HDD / x3 Samsung F3 1Tb / Sony AD-7700 DVD-RW / Auzen X-Mystique / Enermax INFINITI 650W / Lian Li PC-7FNWB
/ Logitech X-530 / Logitech Kboard & MX620 Laser Mouse / Epson RX685 / DELL U2312HM 23" IPS TFT / MY RIG HERE