Alright one question at a time...
About the two motherboards: there isn't that much difference between them. I'd look at the DS3
instead of the S3, as far as I know it's an upgraded and improved version. They are the same chipset, same socket, and pretty similar in terms of overclocking (which you say you won't, but probably end up doing. :P) If you take away the overclocking features of the Gigabyte DS3 though, I consider it just an average board. I like the layout of the P5B-E, as well as the BIOS, the Asus customer support, etc. But you'll have a good buy with either of the boards.
Now, both motherboards can hold quad core processors, so that shouldn't be a reason to pick one. All I can say is I've got a P5B-E and I'm very happy.
Now, about the memory. The speed that the memory will run at default cpu clock speeds will be 533Mhz. So you'll only notice significant improvements once you start overclocking. If you don't plan to, then just get whatever speed you need, since it won't go any faster than 533Mhz anyway. Plus, you can overclock the memory a bit if needed. If you want to overclock you might as well jump right to DDR2-800 since 667 isn't worth it, if I'm to believe the benchmarks.
One other thing. If you're looking to order this baby in the spring then you'll be in time for the next batch of Intel processors, the E6320 and E6420. It has twice as much cache available which you will only notice slightly, but it's still worth it since you're not ordering today anyway.
The memory is compatible with the P5B-E, which some consider a picky board when it comes to memory brands. Since I own it I've thoroughly studied what brands it can take, and Corsair is the most recommended brand for the P5B series I believe.
The 430 watt of the power supply is more than you need. A fully stressed system like that requires about 300W max. The extra 130W is just a little safety and future proof.
My pc runs games perfectly. I can run games on 1280x1024 high settings with good fps (100-200) on the games that I play (CoD2, Oblivion). I do have it overclocked a bit, it does help a few fps for games.