1st Test computer
Athlon 64
Nvidia 8800 GT (AGP)
Any OS option
2nd Test Computer
Turion X2 64x
Radion HD 3200
Any OS run in Virtual Box only
Actual machine after choice is made
Phenom Black 9950 x4
Gigayte HD 6450
Actual Machine 2
Phenom 9950 x4
R HD 4200 + 4700 Hybrid Crossfire
Actual Machine 3
Phenom II x6
R HD 5870
I also have tons of old hardware Mainly P4 Dells that I have configured into a cluster at one point but the electricity use for what I was doing at that time was very much not work the effort. But I might do it again for the fun and challenge if I like mining.
I have GUI Miner on Actual Machine #1 I just couldn't figure it out and when looking for the new version I did not see the link anymore.
dont take this the wrong way, but even with those specs you'd still be at about 90 days average time till finding a block, and the difficulty will change a lot in that time. I'd still recommend a pool.
the R HD 6450 will get you about 130 mhash, we'll call it 150 to be nice. the R HD 4xxx series are only beta-compatible with the SDK, and don't post nearly as good speeds (20-40% reduced over the same model from the 5xxx series -- trust me I learned the hard way). 200 mhash combined for this machine. r hd 5870 is the best model you have here and can be pushed to about 400 mhash = a combined strength of 750 mhash/sec. a quick visit to the calculator over at bitcoinpool.com means you're in for a rough road. I'd HIGHLY recommend a pool. you can dare to be great at a 50% chance of absolutely nothing happening, ever, or you can settle for .5 btc a day (at that rate are present difficulty) in a pool, and at least have something to show for it after a month of work.
I'd recommend not considering solo mining till your rigs combine to the 4 ghash/sec neighborhood.