an upgrade

At the moment my computer specs are:

M/board: ASUS P5VDC-MX
CPU: Intel Pentium D 840
RAM: [unknownBrand] DDR2 2gb
HardDrive: Western 320gb
VGA: ATI RADEON 9600PRO
PSU: 230W

I want to upgrade the following,
M/Board: ASUS P5N-E SLI or GIGABYTE GA-73VM-S2
CPU: Intel Core 2 Duo E8400
PSU: Antec Basiq 550W ATX Power Supply
VGA: GeForce 8600GT PCI-E DVI-CRT-HDTV 256MB DDR2 [already have]

Is this upgrade/build good? or is there other things to buy instead.
i dont want to spend too much on upgrading only around $450 on my computer.

Is there any other motherboards u recommend and any other upgrades should i do?

Thanks