Dell Dimension 8250 video card upgrade?

I own a Dell Dimension 8250, and have upgraded the memory and processor. Haven't heard answers from a previous post, and are anxious to hear if upgrading to an Nvidia GeForce 6200 AGP card would be a good idea (what little life there is of the hd). My question is will 512MB of graphics memory alter my current RAM while multi-tasking, and/or prematurely wear out the motherboard faster? Will a 256 MB card or less work any better since I'm currently only running a 64MB card and a 250w power supply? Thank you