For your situation, the main concern for cooling a pair of EVGA GTX460 768MB SC is, IMHO, not
that they will sit too close to each other (because your motherboard (MSI P55-G80) has one empty slot between the 2 cards.
The main concern should be the fact that the 2 cards use internal heat dumping design, exhausting about ½ of the heat inside the case. If this heat dump is not managed well, it will be recirculated back into the cards and might go through the CPU heatsink fan as well. Obviously, this is not desirable.
Additionally, the cards dump this heat via the front. So, you do not want a strong case front intake fan as its air current will run against the hot air exhaust.
For a low cost
case upgrade, I suggest CM Elite 370/371 Black
. These 2 cases have 2 side panel fan openings as well as 2 case top fan openings. With such features, you can setup a 'top-exhaust' solution (ie removing all those heat dump via the case top extracting fans).
CM 370/371 for US$50 + $17 shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16811119241
If your budget is higher, consider the CM 690 II
. Even the basic version (US$80 + $12) is fairly good already.http://www.newegg.com/Product/Product.aspx?Item=N82E16811119215
For a PSU upgrade, your current Logisys might have enough wattage but it lacks 2 more native 6-pin PCIe connectors. You might try to convert two 4-pin Molex to one 6-pin PCIe connector but I would rather suggest you to upgrade to a PSU with four such native connectors (eg Corsair HX650 for US$100 after MIR: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139012