Jet320 - even though the Core2Duo CPUs use the same socket 775 as your Pentium D, Intel changed the electrical specifications when they introduced the Core2Duo, so chances are 99% that your MB can NOT be upgraded to a Core2Duo. Your only option would be a faster Pentium D, but you've already got a 3GHz Pentium D, so performance improvement probably would not even be noticeable.
Another thing to watch out for is that store-bought PCs like yours often come with the cheapest power supply barely sufficient for the purchased configuration. An 8800GS graphics card (which is very nice, by the way) will require a fair amount of additional +12V power from the PS. It's more likely than not that your current power supply would not be sufficient to power an 8800GS, so you'd also have to upgrade the PS. This Antec is of decent quality and has enough power for almost any future system you might imagine: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371002
This PS is a real bargain (after rebate), but only has 2 of the 4-pin peripheral connectors (i.e. the connectors used to power your hard drive, optical drive, and perhaps some other items) -- if you need more than 2, you'll have to spend another $5-10 on a splitter cable to get more such connectors. http://www.newegg.com/Product/Product.aspx?Item=N82E16817341012