Sorry for my initial bluntness, I just wanted to be clear that trying save $ and work with the mobo, CPU, and RAM you bought would not be a good technical or financial decision. I would hate to see you waste money on a final setup that would not be able to deliver a good experience. If you build the comp I suggest below, the responsiveness and speed will blow you away. Every time I use my mother's, comp (similar to your setup) I get agitated waited for it to move.
Well, you were right in thinking 775 is better, but it is really only superior if you take advantage of it by putting a newer processor in it. The mobo you have there seems to only support single-core procs. Vista, which will be fine for what you do, is optimized for dual core so it will give you much better performance.
Here is my suggestion: I am going with a Pentium build, because I can personally vouch for the components I recommend. However, there is a lot of sense in going with the AMD X2 240 because it does use a newer CPU socket, so upgrading is in. However, if you don't plan on doing gaming or time sensitive video encoding, I am not sure you will need to upgrade. The E5200 will run HD video and multi-task like a champ. If you want to go AMD, head on over to the CPU forums and get a recommendation on a good budget mobo for that.
The System (I highly recommend
www.newegg.com for all parts. Great customer service and low prices!)
$65 - CPU: Pentium e5200
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
$50 - Mobo: Gigabyte GA-G31M-ES2L
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
$50 - RAM: 2 GB DDR2. Yes DDR3 is faster, but for what you do it is not important. (An AMD mobo may or may not support DDR3)
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
$55 - Hard Drive: Spinpoint F3 500gb
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
$40 - Graphics Card: Radeon 4350
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
Total for above: $260
Make sure to buy an anti-static wrist strap for building (<$5 at NE) and familiarize yourself with the process itself. Also, I recommend getting an aftermarket cooler for your CPU (Freezer 7 ~$35 at NE). To get the best performance you will want to overclock the E5200 to ~3.5 ghz. This should give you great performance without serious risks. Consult OC forums for assistance. Also, make sure the PSU is solid, a failure there can damage other components. Hope this helps!