My GFX card has 256MB of RAM and is quick. Can't see the point in wasting all my old drives, saves copying everytjhing off them!!
What card is it? Being AGP, and being surprised if you paid £300 for it, I am guessing it was a low to mid range card about 3yrs ago. So very dated now.
As to the drives, they are simply not worth using if they are PATA. Get a decent SATA.
Keep the old PC as is, give it to kids. Simply not worth compromising a new machine with old parts, as that would throttle it badly. I mechanical terms, running a MV6 on an AR25 box - all that extra performance wasted.
Knowing you do video editting, probably best stick to Intel. The very fastest Core Duos are capable. However, for this task, the fastest P4s are still worthwhile. AMD's do well in benchmarks, but not so well in real world.
For Intel CPUs, Intel chipsets are hard to beat.
For AMD CPUs, Nvidia nForce chipsets are capable.
Hope that helps.