Would try to grab some ddr ram of a friend who no longer uses it and give the machine 2gb of ram. I guess lots of people have unused DDR ram lying around after xmas.
Sounds like a pretty nice system, if its a slide show (and not the ram) then you need something like 100% more power to have it work better. Consider a real slide show goes with 10 slides per second, now if you get a 20% improovement thats still 12 slides....
Also 6800GT is pretty good, you need at least a 7900 to make it better in nvidia terms, if that eaven is the problem.
I more like think that the PC is loaded with other tasks or there is a performance problem, so you can fix that on the software side with no costs. Just make sure not to mess all up... make a backup of all data on the pc first.
Also might help to defrag the hard drive and see if its still working without trouble and fast enough (sisoft sandra hd bench etc).
Another point is you might want to check how many background processes, worms, spyware, adware is on the pc. It might be infected and using 80 percent of its time with malicious tassks.
Or kill the 50 processes running. If the pc has to run email, yahoo, skype, irc, webbrowser, dvd rip and mp3 together with sims it might get slow. Especially video stuff, or downloading mpgs or running file sharing tools in the background.
ciao schutt