Buying any new upgrades with your current configuration will put you in a power struggle. The Antec TruePowerII 550W is extremely reliable. I have had mine for a very long time and it has not failed me once.
(LOL on my old computer I had an 300W psu powering a 9800 AGP and a 2400+ XP. LOL)
AMD Phenom II X4 945 Deneb 3.0GHz | Gigabyte GA-MA770T-UD3P | Crucial 2GB DDR3@1333 | BFG GeForce 9800GTX | X-Fi XtremeMusic