I'd avoid both the ECS mobo and the sempron. I'd also avoid the socket 754 motherboards.
As long as you're buying everything new, try to find a way to get a socket 939 nforce 4 based system within your budget. If you buy at the low-end of that level of system, you can upgrade cpu, memory, and vid card in a couple of years and possibly save an entire upgrade cycle.
See if you can get a barebones socket 939 system. I've seen them as low as $500-$600 for a case, mobo, cpu, hard drive, and sometimes even a basic video card. Try tigerdirect and price out their barebones systems. If you get the right components, you should end up with something that is inexpensive but upgradable.
IMHO.