how about buying BTC directly?
if this is not an option for you,
do some simple math
with 6000 USD, you will get approx 12 7970 @ 400 USD each (or even cheapeer)
these cards spread on 3 or 4 mobos so you will need
3 or 4 mobo
3 or 4 PSU
3 or 4 RAM
you can run the system on usb linux, that costs next to nothing.
add the original 2 7970 (7770 becomes trivial in this setup)
you get 14 7970s, and each can hash at 700Mh/s (correct me if I am wrong)
that gives you 9.8G hash rate.
you have to work out your power cost, and where ever you put your rigs you have to make sure there is enough AMP for these rigs to run together.
so the only unpredictable factor is the difficulty level, with the current difficulty level you get approx 1BTC per day, till the next retarget date
I'd have to agree. With 6k buy btc directly. Think ROI, you'll be making money, to pay for your investment, and once ASIC is fully deployed (BFL finally ships) then I see the difficulty level doubling within the next 4-5 months, meaning you'll be making 1btc a day and then starting to go down....
I have my GPUs mining because I already paid for them since last May and because the price increase made the GPU miners more profitable than before, so no matter how little I mine, I get to profit from it.
6K at actual prices is not too late to get in... especially if you're in for the long term. If you are thinking about buying GPU, then you're in for the long term so buy BTC and profit from the spikes that will surely go up.
I'll use one of my miners as example:
Rig 1: 2 x7970 1x5750 = Total 1500 Mhash/s bought in August 2012 spent about $1200 (bought expensive case, PSU to keep them)
BTC Price in August: $10~
BTC Amount in August: BTC120 (U$S 1200 / $10)
Current BTC generated by Rig 1: 82 BTC as of today.
Total Value of BTC generated by Rig 1: 82 x $70 = U$S 5,740 + Residual value of hardware ($800~) - Power (0.06Kw/h)
U$S 6,540 - U$S 250~ = U$S 6,290
Total Value of BTC if directly invested in BTC instead of mining: BTC 120 x $70: U$S 8,400
I hope this provide you with a clear example of why investing in BTC might be a better option, however, this example applies to my particular case and the value of bitcoin has changed dramatically. I do not sell my bitcoins and I'm planning on keeping them for a long term and I store them offline.