Definitely go for 8gb. its cheap and 64bit windows seems to run much better with it than 4gb.
Yes move your 1Tb drive over and you probably could resue your optical drive if you wanted to also.
dump that motherbd and get something a bit more current.
Heres an 880g chipset mobo for $55 with sata3gb/s and usb2http://www.newegg.com/Product/Product.aspx?Item=N82E16813157292
Heres a 970 mobo with sata6gb/s and some usb3's for $85http://www.newegg.com/Product/Product.aspx?Item=N82E16813157280