I would go with a dual-core processor if I were you. I hear AMD is doing some huge pricecuts on their X2 dual cores really soon. Another good CPU to look at is the AMD Opteron 170 (dual core as well). Its not very pricey, it runs very cool (mine OCed to 2.4 runs idle at 29C and load at 34C in 70F room temp), and OC's like a beast (starts and 2.0ghz, can overclock to 2.6-2.8 on air and up to 3.2 with better cooling and stay stable)
As for a Graphics card, I run an x1900xtx. You can find an x1900xt for about 400. The Nvidia equavilents are much more expensive. If your thinking about an x1800, just go one higher to the x1900xt (big jump in pixel processors and other stuff). Otherwise, just find something better (and that has SM 3.0) thats in your price bracket.
Ram is a good thing as well. Im running OCZ gold edition DDR 500 (really good [the DDR 500 part], best except for one, maybe two other types above it, but much more expensive) unbuffered. I got them for about 175 for a dual channel 2 gig kit. Plus they're shiny and gold.
Motherboard is something to be considered (but probably not as much as others). Just get something that wont bottle neck your CPU and GPU and is compatible with all your other parts.
HD varies. SATA is always good. Im running SATA 3.0gigs (it has about 320 gigs on it)
Power supply only needs enough to power everything, but a little extra never hurt. Make sure you get a reliable one though
Heres my system posted and links
CPU: AMD Opteron 170 @ 2.4
http://www.newegg.com/Product/Product.asp?Item=N82E16819103586
CPU HS: Scythe Ninja
http://www.newegg.com/Product/Product.asp?Item=N82E16835185001
GPU: x1900xtx
http://www.newegg.com/Product/ProductList.asp?DEPA=0&type=&Description=x1900xtx&Submit=ENE&Ntk=all&N=0&minPrice=&maxPrice=&Go.x=16&Go.y=29
Mobo: ASUS A8N-SLI Premium
http://www.newegg.com/Product/ProductList.asp?DEPA=0&type=&Description=ASUS+A8N-SLI+Premium&Submit=ENE&Ntk=all&N=0&minPrice=&maxPrice=&Go.x=15&Go.y=29
HD: WD Caviar 16mb cache 320GB 3.5" SATA 3.0Gb/s
http://www.newegg.com/Product/ProductList.asp?DEPA=0&type=&Description=WD+Caviar+16mb+cache+320GB+3.5%22+SATA+3.0Gb%2Fs&Submit=ENE&Ntk=all&N=0&minPrice=&maxPrice=&Go.x=14&Go.y=29
RAM: 2gigs Dual Channel ddr 500 OCZ gold edition (unbuffered)
http://www.newegg.com/Product/Product.asp?Item=N82E16820227219
PSU: FSP Group (Fortron Source) 700w
http://www.newegg.com/Product/Product.asp?Item=N82E16817104015
Remember, OEM's are not bad, it just means it doesnt have a retail box. Almost all the parts I got were OEM, they're cheaper.