AMD Sempron 3100+ 1.8GHz, 256KB L2 Cache Socket 754 Processor - OEM
ECS "755-A2 V1.0" SiS755 Chipset Motherboard for AMD Socket 754 CPU -RETAIL
these two DO NOT support Dual channel RAM, socket 754 never will, even if the motherboard does, the CPU was not built for it.
so then, there was no point in adding this ram, was there.
Corsair XMS Extreme Memory Speed Series, 184 Pin 256MB DDR PC-3200 - Retail (x2)
please do not post unless you are absolutely sure what you are posting is true, so it doesn't confuse the poeple who are asking for help. sorry if this sounds like i'm having a go at you, it just makes it easier to understand for everyone.
now, back on topic.
Video Card: $209
Hard Drive: $67
TOTAL of:$577, so it fits nicely into your budget. plus a cd or DVD drive and maybe a bit over $600 but thats still 200 under your limit.
the idea is that you get a lesser CPU and overclock it to the standards, and often above, of a greater CPU. this one overclocks nicely, just talk to Nubius
that is basically it for a basic setup - plus a CD or DVD Drive, which is cheap, and you have yourself a very nice system which will play even Doom3 on very nice settings.