I would like to see this answer. I didn't know that you can do what you are doing. I thought the cards had to be in the same series like 5's on 5's 6 on 6's but again I am learning new stuff everyday. I really don't know and I would like to know.
you can't cfx a 4850 and a 5670. both cards need to have the same first two digits e.g. 68xx or 69xx. you can cfx a 6850 with a 6870 (the 6870 will match the 6850's specs) and run them as two 6850s in cfx.
you can still use either the 5670 or the 4850, your choice.
for your setup, you should get a 6850 or 6950/ gtx 560 or 570.
no there's no hack afaik.
crossfiring them is impossible because they are two very different cards. mostly because amd doesn't allow it. if they did, no one would buy their new high end card, instead people would recycle old cards, crossfire them with new entry level cards for higher performance.
use either one of the cards.
or get a more powerful single card like 6870, 6950, gtx 560, 560 ti, 570. or wait for the upcoming radeon 7000 series cards or nvidia's kepler 6xx/7xx series. they'd be much better than the 4850.