A score of 6451 on 3DMark06 isn't that bad with your old machine. The video card surely helps, but there's only so much it can do. You might do better by running 2 gig of ram, but can't say for certain. Also, you don't mention if the CPU is overclocked or not. If that 3800+ 939 is running at stock speed, its definitely slowing the machine down and affecting you score.
That is definitely on the low side with that card. It might be a lack of power. Have you had any freezes or reboots? I would use a program to make sure you have removed all remains of the previous driver. You might try a better PSU if you have a friend that you can borrow one from to see if that is the issue.
-Your CPU will probably hold that card back a little on certain things
-Your 1GB of memory might hold certain apps back as well, current games especially
-You say "plain 450W PSU" what is "plain" and what specs does the "plain" brand have marked on the unit? Specifically the 12V rails and Amps they can handle.
Also make sure you have your graphics card plugged to the PSU and its not powered only by the PCI-E slot.
I know it sounds dumb, but I've heard of people not making that connection (by mistake) and still being able of using the graphics card but with much lower performance. Some motherboards have an aditional 4 pin power slot for powering the PCI-E 16X, make sure yours isn't one of those.
So it has a single 12V rail rated at 18a? That is a problem right there. It could not be getting enough juice and could potential damage other components. I believe yo uneed 26A for the GT but not 100% sure on that.
To be honest with you jay2tall i dont like it either that i am using this psu and i knew that it would have some effect if not a lot towards the bad score but at the moment i cannot afford a new psu.
Plus i have heard people having good scores with the default 240W shuttle psu along with a stable system. So i somehow thought that i would not be that much of a problem but as it seems i was mistaken....