Just drop the memory to around 400, boost the clock to 860-870, and throw it on a sempron machine with a 990fx board, my gaming machine (not a sempron, but a 2500k) has one in it, it's pulling 391 Mh/s on each gpu, slightly undervolted, temps at <80c using Phoenix (aoclbf because it's easy) Real-world income is about 1/2 btc per day with just this card. That is not taking into consideration power cost. Current rates suggest that you would be pulling in an average of 75 bucks a month, minus about 15 for power, I think you could pay off your 60 a month, but just barely.
that sounds nice but with a 6990 you have to keep the mem within 125 mhz of the core, so for instance if the clock was 800 the mem would have to be 675, it is possible to hack the bios and change this but - it is not recommended unless the OP is very experience with this type of thing
ive got two nearly new 6990s I might be interested in selling btw...
I just use sapphire trixx... works like a charm, clock at 860, mem clock at <400, no bios hacking necessary. It is a diamond reference card, I don't know if it matters, but AUSUM switch is in default position, no bios changes have been made. If I could get a 6990 for right around 450-500 I would be interested. I honestly think that a 7970 would be a better option, people are pulling 750 mhash from less than 200 watts of power, some people are dumping them on ebay right now for around 450 since the release of the gtx 680's. Might be better to hedge your bets on a newer card since it will still be relevant for gamers if bitcoin mining tanks for gpu's, but with the butterfly labs situation being what it is, I think we're still a good 6 months from that being a concern, almost long enough to get your money back out of a 6990, but then again radeon 8000 series seems like it is just around the corner. Hard to know which trigger to pull on a used gpu for mining atm. Personally I like the unlocked 6950's, I've picked up a couple with 2+ years warranty left on ebay for right around 200 bucks each, one a gigabyte reference model 2GB and the other an asus dcII 2GB. I will send a screenshot if you would like for verification purposes.
Would be awesome to make some money over here in The Netherlands. I have 2 dedicated miners, but 0,30$/KWh prices are killing me =[ 