You shouldn't need to pull off the shroud, on one side there are clips that will release when you lightly squeeze the shroud just above them, then lift that side and it should open.
While the chipset itself will support the following CPU's, the motherboard may not (most worried about the quad core) be able to deliver the power needed to run them properly
For Core 2 Duo CPU's the best is the E8600 which runs at 3.3GHz and uses 65W
For Core 2 Quad CPU's the best is the QX9650 which runs at 3GHz and uses 136W (I highly doubt the ability of your board to handle that)
Checking out upgrade charts, I'd say your best bet would be to find a Core 2 Duo E8500 3.167GHz at 65W which seems to be a happy CPU regardless of what Q45 chipset motherboard it calls home.