Windows 7 - Building a "B-Rig", help needed

Hi, I'm planning on building a secondary, work-dedicaded PC. I'll start it with leftover parts from my "A-rig", but I'm still lacking essential parts. Now, my dilemma is wheter to upgrade A-rig, and use the old parts for the B-rig.
Or to simply buy new and cheaper ones for the B-rig.

The B-rig is going to be wholly dedicated to photo- and videoediting, but I don't need to have anywhere near the performance of the A-rig, but it's a plus
No need for gaming performance (that's what the A is for )

I don't have a set budget for this, as I'm still assessing what this will cost, but I've estimated it to be around 400-600 euro (dollar). Is this realistic? I'd like this to remain under 800 euros, but the guiding line is quality/money.

I don't need top performance right after building it, but a good basic setup (mainly CPU and MoBo) which can be updated when the need comes.

My current setup:
Case: Silverstone TJ05
CPU: Intel Core i7 920
GPU: Geforce GTX 570
MoBo: Asus P6T SE
RAM: 12gb 1600mhz Corsair (6x2gb)
HDD: OS 1x 80gb Intel X25 SSD, 2x 1,5tb WD in RAID 0, 2x 1tb Samsung F3 in RAID 0
PSU: 650W Corsair
Windows 7 Home Premium

Leftover parts for the B-rig:
Case: Antec Three Hundred
GPU: Asus Geforce GTS 450 or ATI Radeon HD 4890 (Sapphire)
HDD: 2x 500gb HDD, 1x 40gb Intel X25 SSD

So, I'm still in need of a CPU, MoBo, RAM and PSU.

I hope this covers all the basics, thanks for any assistance!