Wow - a £200 card for your kids PC - lucky things!
Anyway, loads of graphics card reviews can be found
here including a round up of the latest GeForce 6800s - tested with a 3.4GHz P4 and AMD Opteron (the ATi/Intel & nvidia/AMD compatibility thing you mentioned sounds a bit dubious IMO). Basically, any half-decent card you buy today is gonna be a WHOLE lot faster than the geforce 256!