Of course you would see some difference. The question should be whether it is worth it.
I see your biggest improvment being on your FSB. You will be moving from 400mhz to 1000mhz. After all, it doesn't matter how fast your memory and processor are if they can't communicate fast enough.
Overall your talking about completely technologies all around. AGP is in the past, so even if you get what would appear to be a similar graphics card, the PCIexpress will perform better.
I don't see 2GB being much of an improvment over 1.5GB though, especially if you're just running one game at a time. Memory->CPU connections just aren't fast enough.