I made your move recently. A q6600 to i7 and Windows Vista to 7, but had an SSD for one year:
My complaint is that in the develop module it takes about 5-10 sec. to load an image. In PS CS5 the panorama stitching dialogue occasionally crashes with an error messages that there is insufficient RAM.
Right click the taskbar and go to the Processes and Performance tabs, this will tell you how much memory you're using and what programs are taking the most and the reason for the crashing. Vista is resources intensive - if you've combined an antivirus program with whatever programs you've installed, you're in trouble with a 32 bit OS that can only handle 3+ GB of memory. (Note you can turn off many of these programs running in the background in: run-> msconfig-> then click off the stuff you don't use in the startup and services tab - remember to check 'hide all Microsoft services' and document all the changes you've made in the case you've turned the wrong thing off).
My questions are:
1. Can I speed up the LR develop module simply by buying a SSD and putting the ACR-Cache on that disk or do I need more RAM/CPU power?
Your stuff loads almost instantly with an SSD, write speeds are not as quick.
2.If an SSD doesnt fix my problems, would it be sufficient to upgrade my CPU to i5 2500, the RAM to 8GB and windows 7 (64 bit) or would I need the SSD in addition to that new hardware, to gain anything over my current system?
Your current system is pretty powerful. The SSD should be enough with Windows 7.
3. In other words: Does lightroom gain more speed from a better CPU and Ram or from a faster disk? At this point I cant afford both.
I've forgotten how long I use to wait for things to load back in my pre-SSD days (O.K., I'm not telling the truth - I suffer at work with a regular drive). Skip the CPU.
Really, you might be fine with just installing 7 (as it uses less memory then V) and the cool thing about is it comes with both 32 and 64 bit versions (side note: I upgraded to 9GB of RAM but still haven't used more that 3gb at one time - would change with video / music work). But really, I'd go with the SSD + a large capacity H.D. or two (and a second for backup), install Windows 7 try 32 bit first to see how things go. If you still get the memory error (you probably won't), make the jump to 64bit and be prepared for a driver / program problem (or two).
If your manual says 4 Gig max, maybe that's a OS limitation, not one of the Mobo.
I suggest upgrading to Win7 64 bit, then see if you can install two 4 GB sticks. If it works then that should help a lot.
If that doesn't work then look into upgrading the Mobo. I'd keep the CPU - it's pretty nice and it'd be a shame to toss it.