To Paul:
Firstly, since you will install this video card onto your bro's system which is a P3 system, AGP 8X wouldn't do you any better (sorry I totally forgot about this). So maybe you can go for cheaper models.
As for memory size, up until now I don't still see why video cards need such a hugh memory nowadays. It wouldn't boost the speed much at all. It is so-called 趨勢 ba... XD Simply go for 64 (32 is out dated XD), 128MB is totally un-necessary.
To me, video memory is only for resolution output. The more the memory, the higher color and higher resolution you can output, that's it.
nVidia has a variety of GeForce series video cards, you would have to study more about the differences between them. I haven't really looked into this for such a long time, so... I am afraid I couldn't give you a very precise recommendation...
To Jin-Ron:
I strongly recommend you to take it to a system-pro (which I am not XD) to have your system examined. I don't know which mainboard that is, but I use the same video card as you and I don't have the same problem... Sorry that I couldn't give you further help... >_< Try what Aced said ba...