I am going to be purchasing a nForce mobo in the coming weeks and I have been contemplating this very comparision. First of all I'm gonna say that you it seems need to use 2 identical sticks of DDR on nForce mobo's, So, you would have to use for example 2 sticks of 128MB of Crucial PC2100 DDR for Twinbank (I've purchased 2 sticks of 256Megs of Crucial PC2100). You must use identical sizes and makes. Now on to the 2 mobo's.
I will start by saying that the Non-E version of the A7N266 should be avoided. It does not overclock any better than the K7N420 Pro, and does not have the 5.1 DD nForce Audio (It uses a C-Media Sound Chip) and does not use the on-board Ethernet. The K7N420 Pro is a solid board, and a great value at $150 and really, I would have to take it over the A7N266-E if you aren't going to be overclocking. If you have any intention of Overclocking, then the A7N266-E is well worth the price. Also if u need the extra sound ports than the A7n266-E is better as well. It really comes down to are you going to overclock? Here's the overclocking results on the 2 boards (K7N420 Pro's results are from Ace's Hardware review, and the A7N266-E's are from [ H ]ardOCP's review of it)
K7N420 Pro while using Integrated Video: 154MHz fsb/mem clock
K7N420 Pro while using a GF2 GTS in the AGP slot: 145MHz fsb/mem clock
K7N420 Pro while using a GF3 Ti500 in the AGP slot: 140MHz fsb/mem clock
A7N266-E while using a GF3 Ti500 in the aGP slot: 158MHz fsb/mem clock
So if you plan on overclocking, then take the A7N266-E, but otherwise (except for if u need the audio jacks) the K7N420 Pro is a fine board and well worth the $152. Good luck!