I wouldn't recommend buying a new 250GB hard drive for a desktop PC. You can get larger (500GB or more drives) with higher performance for about the same price, or less.
For the RAM, get a 4GB dual channel kit (2 X 2GB) for the best performance.
If the system is for gaming, the Radeon 5450 is supposed to be pretty weak. Unfortunately, good, cheap (< $100US), gaming graphics cards may not exist.
The motherboard that you list is a good one, but expensive. You can get AM3 boards for a lot less money. (Avoid the ones with nVidia chipsets, which seem to be kludged up AM2+ designs.)
The CPU is OK, although you could get an X3 450 with a higher clock for only a bit more.
You might start here: Budget System Builder
It mentions some additional factors, like cases and power supplies.
I also suggest familiarizing yourself with the "power search" engines at www.newegg.com
. There's a lot of information there, even if you can't use them (they only serve the US and Canada) or don't want to deal with them.