Well I do a lot of the same kinds of tasks as the originator of the thread.
I run two Q6600 based systems, both with 8GB RAM, both on ASUS P35 motherboards (P5K-Deluxe-WIFI; P5K-E-WIFI). Both have 8800 series graphics cards (8800GTX, 8800GT-512).
I would not for one MOMENT suggest doing anything BUT getting the Q9450 quad core CPU when it is out for $320 USD (or so), and ALSO getting 8GB DDR2-PC2-6400 RAM.
Honestly if you've outpaced the capacities of your current system, are doing serious memory / CPU heavy computations, and can spend the money, at the current performance / price of the Q9450 and 8GB RAM it's just incredibly attractive and PRACTICAL / USEFUL to get both.
The RAM selling price is basically right around the manufacturing cost of the parts due to the market glut of DDR2 RAM. It's as close to cheap / free as it is likely to get for years to come. And it makes a BIG difference as long as you have a 64 BIT OS like LINUX, SOLARIS, BSD UNIX, VISTA 64, XP PRO 64, etc.
The Quad Core CPUs are also INCREDIBLY good values for the performance they give; only a few months ago the Q6600 was selling quite well at over TWICE the current price, and the Q9450 will be at least 25% better in many cases for actual performance (SSE4, VM, better cache, better architecture).
Though not all SINGLE applications USE multi-core CPUs, don't let that fool you, if you're running the OS that is going to efficiently use one or two cores just for itself, even the most single threaded application will use one more core, and if you're running ANYTHING else like printing, a database, a web browser, etc. you'll make good use of another core or two.
And, frankly many of the better imaging / GIS / database / mapping type applications WILL take direct advantage of more than two cores already.
I see the smoothness, responsiveness, speed, and reliability benefits of running quad core + 8GB every day.
I have good things say about both the P5K-E-WIFI and the P5K-DELUXE-WIFI; I'd tend to suggest the P5K-DELUXE-WIFI with the Q9450 since you're more likely to be running at higher FSB speeds with the Q9450 or even one of the new E8xxx dual core CPUs if you overclock at all versus the lower FSBs / lower overclocks on the older generation of CPUs.
Thus the better heatsinking of the P5K Deluxe may be of benefit, though either mainboard should be a reliable choice for you.
I've installed VISTA 64 and LINUX X64 on these systems and things work with very few problems. I would not be strongly tempted to go with another OEM's motherboard. Some of the X38 chipset models may be reasonable considerations, I don't personally see that they're necessarily better than the P5K-Deluxe for most purposes.
You'll end up with a system that'll handle pretty much anything you can throw at it for 5+ years to come, and at a price less than what a high end dual-core 4GB system would have cost you 9 months ago.
I don't think that the VM advantages of the IOMMU equipped Q9450 and the various other new Wolfdale/Yorkfield dual/quad CPUs will substantially benefit you in the short term if you're not using VMWARE / VIRTUAL PC today. However I think it will become more of a prevalent benefit within the next couple of years as VMs become less specially orchestrated configurations and start to transparently be employed within the OS and within applications themselves. The new IOMMU/VM technology is MUCH better than the previous generation hardware since with it you can meaningfully virtualize the hardware peripherals as well as the software programs.