First, ignore this moron. Everyone knows 1 GB is more than enough memory for a single core PowerPC notebook. Memory isn't the issue.
I think it might be more than your HDD being full/fragmented, there's a pretty good chance that it's dying; as mentioned, replace it.
Get a nice new hard drive (obviously you'll need to get an ATA instead of an SATA HDD), and you'll probably notice a world of difference.
The fact that it's slower than before could have to do with the fact that you just reinstalled and Spotlight might be running. Have you checked that? Spotlight can tie up even the fastest machine, irrespective of CPU and memory.
Bottom line, NEW HDD. I don't think a faulty frame buffer would affect you like this.
Good luck. (Also, as suggested, do an NVRAM reset... and try a CUDA reset as well.)