-Are you running retail games? They are pretty old, and your GPU is pretty new. I wouldn't be surprised if some driver issues were making the games lag with the new graphics card even though it worked on older card.
-If you insist on playing the retail games, check what sound driver you have selected from FreeSpace 2 Launcher. If you have it on Aureon 3D sound or something like that, try the other options (don't remember which options they are, possibly DirectSound) - when I first got FS2, that option made the game lag like hell indeed.
-If you want to enjoy FS2 to the fullest and beyond, download FS2_Open and FreeSpace Port. This will enable you to, first of all, play the game, fully utilizing newer hardware, enable higher resolutions, play the original Conflict:FreeSpace the Great War with FS2_Open engine, many mods that don't work with retail, and if you want you can download all kinds of graphical upgrades for the FS2 and Great War campaigns (though your PC will likely choke on at least some of them). Also, you will get whiter teeth.
Basically, what you need is the following:
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FSOpen Launcher, which will let you select the fs2_open executable instead of the retail FS2.exe;
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FS2_Open build - these are the latest official release builds, 3.6.9, which is rather stable and good starting point for you I would say.
Simply copy the launcher and the executables into your FreeSpace2 directory, then open the Launcher, hit "Browse" on the upper part of the window and select "fs2_open_3_6_9.exe" from the list. Other files in the rar file are a debug build (which you should only use when solving problems) and FRED2_Open normal and debug builds, which you should use if you're designing missions.
Then navigate to Video tab of the Launcher, set the resolution and bit depth and then run the game.
Other option is to use the online installer, which downloads a crapload of cool stuff onto your PC, including the aforementioned graphical goodies, but you might want to first try playing with FS2_Open and retail FS2 media, just to find out if you can get things working that way (and to save time). If that works OK, then you can easily run the fsoinstaller in the directory if you want to see if your PC can handle the MediaVP's. And of course you can use it to get FSPort, which like said is basically a port of original FreeSpace onto a form that can be played with FS2 and FS2_Open engine.
Power supply might be a problem, then again I have a 400W PSU and it seems to manage the power demand of my 8800 GT (and AMD Athlon X2 4800+ and three HD's and an USB sound card) with no problems... things aren't always as simple as looking at the nominal power output of the power supply.
You might want to also check that your card isn't overheating and throttling itself down (which causes graphics to slow down, of course); if the card has passive cooling, makes sure the case is well ventilated, if the card has active cooling make sure that the fan and cooler aren't clogged with dust... and makes sure the case ventilation is OK, although you would likely have noticed overheating related issues affecting other things as well if that was the case.
