Hi! Both are very nice high powered system and you really would be best going for any 4200-128MB. Even the cheapest 4200-128MB using the 'slowest' 4.0ns RAM o/c to around 300/550 and therefore outperform GF4TI4400 (275/550). 4200-64MB cards do use the faster 3.6ns RAm as standard and as such reach 300/620 speeds, but the 64MB is already becoming limiting and a 4200-128MB at 300/550 is still faster, esp in the longer term. If you can find a 4200-128MB using the faster 3.6ns then this will o/c to about 300/620 and give perf VERY near to GF4TI4600! It is very difficult to find the ns RAM used as neither manu nor retailer usually specify it, but this is by FAR the most influential factor when it comes to o/c'ing, manu, RAM HS and fancy HSF designs do VERY little.
If you want a 4200-128MB guaranteed to use faster RAM and therefore come with higher default clocks AND o/c faster then there are currently 2 options out there. They use the longer board design from the 4400/4600 cards and also fast 3.3ns BGA RAM. These cards are 'Asus 4200 Deluxe' and the 'Suma 4200 Special Edition'. They sport 260/550 default clocks (250/444 is usual) but o/c to 300/640 (Asus) and 300/700 (Suma).
There are 3 other options which are about $50 cheaper, but the 4200-128MB is by far the better choice. The first option is a GF3TI200, when o/c'ed it gets to within 10% of GF3TI500 perf (still slower than 4200) but is let down by relatively poor '2D image quality' and single RAMDAC. The second option is Rad9000/9000pro, but since this is inferior to the Rad8500LE/8500 options it makes VERY little sense going for one. So the third choice is Rad8500LE/8500, the 8500LE is about 10% slower than the full Rad8500, these cards are easily on par with the GF3TI500 and sport better '2D image quality', TVout and dual monitor support BUT are very lacking when it comes to AA. Another thing to watch out for on Radeon cards is that both oem (vs retail) and non-ATI ('powered' as opposed to 'built by ATI') use cheaper RAM, lower clocks, o/c poorer and often sacrifice features, build or image quality. So if you go Radeon, be sure to get a true ATI card unless you get a HUGE bargain. The 4200 cards are by far the best all-rounders and certainly have the best longevity. They have better 3D perf, AA, TVout (than GF3), dual monitor support and '2D image quality' (about on par with ATI Rad8500).
Whatever card you go for, be sure to get 128MB RAM on it. In short I am unbiased and in the know, and 4200-128MB is certainly the way to go. Don't be worried about manu for GF4TI cards as it makes very little diff, Gainward and Leadtek are often considered slightly better for '2D' image quality, but unless you can run above 1280x1024x32 at a decent refresh rate the diffs will be unnoticable. So go for RAM type, features and then budle with manu being low down on the list.