Budget cracking machine

Hi guys I'm interested in you opinions what about I'm going to say
I would like to say first that this kind of project is a hobby thing so i would never ever spend thousand of dollars on it.

I wanted to build a low budget cracking machine and here is what I put up in theory.


I searched for second hand pc parts and here is what I've got:

MSI K9ND Speedster2 (MS-9661) ATX motherboard with 2 PCI-e x16 slots and 1 x8 slot

2 AMD Opteron 2352(8x2,1gh) or 2 AMD Opteron 2354 (8x2,2gh)

16gb ddr2 memory

400W Seasonic SS400H1U PSU

Raid card: Areca ARC 1120/1220 256mb 4 port SATA RAID

A bunch of cable for this.

2 fans

this is so far 76 usd or with the better cpus 86 usd

And for the last part the gpu

Radeon 5970 for 69 usd

so the hole config is 145 usd and it can be expended with an other 5970 to reach insane speed for this price.
I'm highlighting that the hole rig is build by used components and I already have an HDD.

So what do you guys think about it? Is it a waste of money? Or a brilliant idea?