Cost-effective upgrade?

Back in June 2006 I built a home PC that would be capable of running Autodesk Revit Architecture software, a 3D design software used at my architecture office (no it is NOT similar to AutoCRAP in any way). It can be fairly resource-intensive at times. My machine's current specs are as follows (everything purchased in June 2006):

Antec 450W power supply
Intel D945 mobo
Intel Pentium D dual-core 3.2 GHz CPU (Core 2 Duos were just coming out back then and were too expensive)
2 GB Corsair DDR2 SDRAM (mobo has 4 slots)
PNY nVIDIA Quadro FX540 128MB graphics card
Windows XP Professional x86

I would like to upgrade to Win7 x64 and at least 8 GB RAM. I know the Pentium D CPU works with a 64-bit OS, but I also know that newer CPU's run faster/better than the Pentium D's, even at lower clock speeds. Many people on the Revit forums report excellent performance with Core 2 Duo CPU's.

So that would mean mobo, CPU, and memory all need to be upgraded. I also need a new graphics card as the digital output on my current one seems to be on the fritz. Can anyone make any suggestions as to the most cost-effective solutions? I have always gravitated towards Intel products, even for the mobo, but I know those are more expensive. I've heard of people using other mobo brands with good results, but I'm just not "in-the-know" on what's good and what's not.

Thanks for the info. I want to say my mobo is a D945GNT-something... not sure right now, would have to check the box later tonight.

For graphics cards, I have always preferred the Quadro FX workstation cards, as Revit never has really played nice with "gamer" cards. Revit doesn't heavily depend on graphics card power as much as other programs like 3D Studio Max, so the entry-level FX 500-series cards usually work fine.