Hi!
So, first off, as others have said, do not worry about the graphics card too much. LR doesn't benefit from one - at least not in the sense of computing effects or doing the exports - and in Vista MS disabled many graphics accelerations for the standard 2D interfaces. Reintroduced this in Windows 7. Still, Windows 7 is basically Vista version 1.5. Most speed enhancements from Win 7 have come to Vista through SP 1 and 2. Apart from that the UAC has been refined and you've got sseveral new features. But basically it's Vista, just more polished (which is why Windows 7 has the internal version number 6.1 and Vista has 6.0 (by the way, Win2k was 5.0 and XP 5.1).
Back to hardware. Basically your chipset should support the C2Q 9550, which is a pretty fast CPU. I do not know about the BIOS settings but it might only be that the CPU is not configured automatically and you need to enter the parameters manually. I had the 965er chipset with this CPU (your 975X is one step above that) and it was quite nice with LR3.
Changing only the CPU will keep you your Vista installation so in that regard it's the easiest way for getting more speed. Even easier is getting more RAM if you have less than 8GB (for proper CPU mounting (or better: for properly mounting the cooler) you need to take the mainboard out of your case. You do not need to do this with RAM.). I have a Pentax K10D and working in the development module on a set of about 400 pictures and applying diverse actions (noise reduction, lens profiles, exposure adjustments, cropping and so on) will usually eat somewhere between 3 to 5 GB RAM. Since my camera has a only a slightly higher resolution you should also be in that range. Remember most people answering here have usually 20+ MPixel caameras so they definitely feel the need for even more RAM.
Adding RAM was the biggest speed bump for me while actually working with my pictures (library and develop module). Going for the 9550 from a C2D 6400 (two cores at 2.1 GHz) mostly enhanced importing (I convert to DNG during import) and exporting the RAW files.
Now, the 9550 costs basically too much in my opinion but it might be your cheapest option if you already have 8GB. If not, well, then you might consider upgrading the whole set. DDR3-RAM is extremely affordable at the moment (here in Germany it's about 50€/8GB). You can get a decent mainboard with a good array of features (including USB 3.0) for about 100 - 120€. An i5 2400 is about 160€ and is roughly 40% faster than the 9550, sometimes even more than 50%. So basically for about 300€ you can get a state-of-the-art system. In Germany at the same retailer I looked up the above prices the 9550 is 309€ to give you some perspective. Of course this would mean reinstalling Vista (you might try to install the new mainboard drivers on your current system for Vista to find once you boot the new system but this might not always work as expected. A clean install would likely be more hasslefree). Oh, and if your optical drive still is IDE you might want to pay attention to the mainboard still coming with an IDE controller. Or buy a new SATA optical drive also.
Of course, the final option would be going the SSD way. This will speed up almost any task on your system (you won't see a difference in Word with a faster CPU but you will see Word opening almost instantly (depending on the SSD) with an SSD compared to even a fast HDD), especially loading times will be greatly decreased. This means your catalog will load much faster and also the previews should appear faster. Importing/exporting should also be faster but for your files probably not really (also see
diglloyd). Working in the develop and library module is mostly dependant on the CPU. And since SSDs are still rathe expensive I doubt you will use them for anything different than the system drive (replacing your Raptor). So importing or exporting will mostly be as fast as it is now for you if you decide to only get a new SSD.
Oh, of course, I you have insufficient RAM and LR needs to use the pagefile than the SSD will be of help, yes. But buying more RAM is usually cheaper
