There is zero practical difference between P43 and P45 in your case. The P45 chipset comes with the ability to have DDR3, usually not implemented. The P45 chipset comes with the ability to crossfire, but you are not building a crossfire system.
However, the UD3R is a very well made, second generation P45 motherboard. It's probably the finest single PCI-E slot motherboard being made right now. It's a good choice, as is it's brother, the UD3P, which is crossfire enabled.
The Thermaltake "W0100RU" is a cheap PSU with poor voltage regulation and ripple. No active PFC. Rated at 70% efficiency but likely less. Doubtful it would sustain 400W output long.
I was thinking of that. Infact I picked almost that same build. I did not like the fact that the 64x2 7750 sucked up 95 Watts and got so many complaints about it's stock cooler. I am not going to toss the idea out the window though. It is a great budget saver.
I have looked at trying to save money with a GeForce 9800GT however their prices really are not competitive at all compared to the 4850s. You get a whole lot more performance just for $10 or $20 more.
This build is for a friend. I think it may be best to push the budget to $650 just to get those extra quality parts.