KB,
Back up all your stuff! I say this because Kaesan has a point. The Dell images tend to get flaky when you change major things and conflicts arise. In that case a rebuild of the OS will result.
Just a handy tip: Partition your drive into at least 2 partitions. One for the OS and one for the programs and saved files. This way you can do backups, but unless the drive takes a sh!t, you don't need to whack all your data, just the OS partition and rebuild from there. Lots less file shuffling.
As for the PCI/AGP thing. With 8x AGP going around, yes AGP will be faster than PCI, but not a hell of a lot.
Memory; Are you sure about that PC1200? It's not PC2100? If you can get faster, I'd say do it, like PC2700. But the more mem you've got the better. No such thing as too much RAM.