compare the RAM of both cards ... in some cases the higher amount of memory does mean a differen type of memory ... sometimes faster sometimes slower .... so search the net for mem timings and speed
and I would take the Abit I really start to get an Abit Fan .... but ASUS always did a good job, too

The Abit is the best, but more of an overclockers board, the ASUS however is a nice stable piece of hardware. Also weigh up the post sales support on the different manufacturers if you're new to this stuff! [easy, just do a search on the mobo and the word 'problem' in google] Since they all feature the same northbridge / southbridge combo, you may also wanna weigh up extra options on the different board [extra USB/FIREWIRE controllers?]

Videocard wise, go for the 256MB option if the price is aobut the same, but as Mertsch said, check out what DRAM chips each is using, having 256MB of 4ns memory is nice, but having 128MB of 3.3ns memory is better! Also, the cooling option is important, in most cases I'd go for Sapphire [better support].

If you're gonna go for a gaming PC, why not go the Springdale [865 chipset] route, save some money and go get a 9600XT which roxx ass beyond the other two cards you mentioned?

Also, make sure you have about 512-1GB of memory if you wanna pull ahead in games, a nice HDD with 8MB cache is also always on my to-buy list.

i believe that sapphire is the best option here (otherwise look for a Tyan)
powercolour isnt known for the best cards ;P

Get the Saphire. Id go with the MSI Board also. If you read the benchmarks on that board / over clocking possiblility it is very awesome.