no 6pins or 6+2 pins at all?, didnt think you could get such a psu today=P
Most people doing mining will recommending using a good psu, preferable a 80+ (Bronze/Silver/gold/platinum) sertified psu

You can use a adapter, but chech that the psu can deliver enough power on the 12v.

Most PSU of higher quality use a single 12v rail for the PCI-e connectors which has available allmost all the power from psu.
Example my Seasonic X1250 psu has 8x 6+2pins and has capability of 1248w on the rail=)
With driving 4x5870 the psu is room temperature and takes it with extreme ease..

Btw the 5870 has a peak wattage usage of 188w, so be shure that the 12v has this capability

I wouldnt say its  a waste. with a 5870 u get around 300hash(stock speeds), thats equals to about 0,039btc/24hours
thats 2,26dollars pr day given the todays marked of 58$ pr btc.

It comes down to what you pay for your power.
Also i wouldnt worry about them asics just jet. When Carpo was online with his 300kHash/s power i got really good payout since the pool had around that time 1.8THash, hit BTC more often and there was a bigger chance that i got a payout=)

The problem with those asics is that the mining speed will go trough the roof and so the bitcoins will be found faster/ergo the difficulty would also go really fast up.
As of now i see no problem mining with GPU.

I have bought 2x7970 2x7850 4x5870 2x5830 within the last month, and im not worried=) also if the bitcoin marked would be "killed" by those asics, you can allways mine litecoins (which doesnt support asics jet, but recently started supporting GPU, previously only CPU).

I do also think that the bitcoin value will go up as the ability to mine them will be more difficult.
Just look at the last month, was around 20$ now it is 58$