You probably want an LGA 775 quad core Intel processor.
The QX9770 is about the best the slot can possibly hold, but you would have to check whether your motherboard can support a 136w processor or not. That would also be the most expensive option.
A Q9650 would also fit the LGA 775 slot and its only a 95w processor so its much more likely the motherboard is capable of running it. It is less expensive than the QX9770, but probably easier to find.
The Q9650 isn't a lot faster in clock speeds than your current one (3.0 GHZ), but the 2 extra cores should be picked up by Sony Vegas and used to very good effect.
The Q9950 would be another option that would be 2.83 ghz per core but with 4 cores. It would have most of the benefit of the 9650 because of the extra cores, but for anything that can only use 2 cores it would be no better than what you have now.
Windows 7 64 bit vs XP 32 bit... I made the switch some time ago and kinda wish I had done so sooner. I am usually one of the slowest to pick up a new OS, because it allows me to skip the worthless OSs like Windows ME and Vista, but for the really good ones like Windows 7 it slows down my adopting a really good OS too.
If you go on Microsoft's website, you should be able to download something called a Windows 7 Compatibility Checker or something that will tell you if your computer will be able to use Windows 7 and if it can there is no good reason why you shouldn't do it.
Being the owner of an XP 32 bit OS, you qualify for a Windows 7 Retail Upgrade CD like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116713
Note that the one you want doesn't say OEM anywhere on it. The OEM CDs cost the same as retail upgrade CDs if you qualify for it, but the license agreement of the retail upgrade CDs is much much better. Indeed the OEM CDs are illegal for anyone who wants to buy it and install it on a computer for their own use.
If you need to use Windows 95 or 98 programs (16 bit), you won't be able to do it with the Windows 7 Home Premium 64 bit OS. You would need to get the Windows 7 Professional 64 bit OS because it has an XP compatibility mode and XP is capable of using 16 bit programs. It would probably be $10 or $20 more for a Professional upgrade CD as compared to a Home Premium upgrade CD.
If you don't need to use anything from the Windows 95/98 days, then Home Premium 64 would do just fine.
If you DO need to use something that old, I would suggest you look for a better alternative anyway, because supporting stuff that old is more trouble than its worth.
Anyway, upgrading from XP to 7 isn't that hard or anything and we can walk you through it if you think it is something you are interested in after you internalize the above points.