Looking to build a new rig but need help

I used to be a member here years ago but havn't been back in a long while. My rig is now 4 years old and I'm looking to build a new but smaller one .
I feel totally out of touch with OC'ing these days so I'm looking to you guys for some much needed guidance.

I'm currently looking to go SFF with:
MSI mPC51V case £166
AM2 Athlon64 4200+ (Energy Efficient model) CPU £106
2GB (2x1Gb) Corsair TwinX XMS2 PC6400 RAM £147
512MB Sapphire RX1950Pro PCI-E Graphics card £143

Is this gonna make decent machine or am I making a big mistake somewhere?

My budget is £550 and this lot just passes that, can I save money on lesser RAM that will clock up ok?
Are there better graphics cards out there for the same money?
I still don't know if 4200+ or an Opteron would be better. Like I said, well out of touch these days.

Please enlighten me guys!
Really appreciate all help here.

I am not familiar with that board model, however everything else looks fine. Have to be carefull about OCing in a SFF setup as heat can be an issue.

My opty is proving to be very nice and have had no issues with it. Dont know whether the extra cache helps anything or not but sure cant hurt.