I'm not sure that you need a new graphics card. Your X1200 doesn't have full Windows 7 (WDDM 1.1) drivers
, but the newest listed drivers: ATI Catalyst
are for Vista (WDDM 1.0). I think that Aero Glass effects should be complete with them.
As for the audio drivers, as IggyAZ points out, uou have a Realtek ALC888 audio chipset. I suggest trying the HD Audio drivers directly form Realtek: Realtek
They list the ALC888 as supported under Windows 7 X64.
As regards a possible new graphics card, yes, you might not meet the recommended requirements with a 250W PSU. The lowest-end DX11 card that I know of is the Radeon 5450; even that wants a 400W supply.