Recommendation on Motherboard and CPU upgrade

Hello,

Looking for some advice.

This is my current PC hardware

Motherboard - MSI K9VGM-V micro ATX
CPU - AMD Sempron 64 AM2 3000+
RAM - DDR2 667mhz 2gig (2 x 1 gig) 240 PIN
Video card - ATI Radeon HD 4650 PCI EXPRESS 1 GIG (I use this as my sound too it acts as a passthrough to my yamaha receiver
Hard Drive - 250 gig Western Digital + 1 tb external western digital hard drive
PSU - Cool Max 400watts 120MM
DVD ROM/BURNER

Now I am debating upgrading my motherboard and CPU or just upgrading my CPU

The link below is my current motherboard specs

http://www.ciao.co.uk/Productinformation/MSI_K9VGM_V__6...

If I were to just upgrade my CPU I would chose the AMD Athlon 64x2 6000+ 3.0ghz (link below on specs of CPU)

COST $80

http://www.tigerdirect.ca/applications/SearchTools/item...

OR

If i were to upgrade both the motherboard and CPU would go with GIGABYTE MA785GM-US2H with a AMD Phenom X4 Quad Core (link below on specs of both)

COST $238

http://www.tigerdirect.ca/applications/SearchTools/item...


The main use of my computer is to watch compressed HD movies (4.5gigs to 8gigs) and play games (I am not a hardcore gamer but do play games like call of duty modern warefare, need for speed ect

Now I do not want to spend anymore then $260 do you think it is worth it to spend the extra money to upgrade both the motherboard and the CPU or just upgrading the CPU is good enough I do not want to change my CPU and then 1 to 2 years down the road I need to upgrade everything.

Any advice would be greatly appreciated Thank you in advance