Dear Francois,
I probably have to shed some light on things that you either tend to misunderstand or don't know much about.
First, i'm a hw tech. I know exactly what components are required for specific purpose. EG. "3 ghz" processor is not like 3ghz processor. Old net-burst P4 at 3ghz will barely beat performance of C2D at 1,8 in most scenarios. I've got quad at 2,4...
"Speedy RAM" is not DDR3. DDR3 has terrible latencies which cripples system performance in real life horribly. The only chipset that handles DDR3 really well is X38, which is very new and only few companies released mobos with that chipset (gigabyte was first i think). In addition, even DDR2 @ 800 is enough for todays cpus. Future intel cpus will have up to 12 MB cache which reduces the need for low latency and Mhz packed RAM even more. Not to mention the BIG BIG price of course and it's not available in normal volumes. Simply, DDR3 is nonsense for at least a year, probably more. Vista was released quite a few months ago...
All in all DDR3 will improve performance a bit, but it's surely not so lethal. Common rule is: it's always better to have lots of maybe slower RAM than having rather small ammount of super-uber-fast RAM. 4 GB is absolutely enough even for x64.
"15 K" drives. Hmm. Riiiight. As already mentioned, every OS (and filesystem for that matter) will be faster with faster drives. That's natural. Vista was not meant to be fast only on 15.000 RPM drives, because it's not one of google servers

And a common user with average budget would forget about any OS that would recommend hardware like this. That's simply nonsense.
10.000 RPM Raptors are still one the fastest non-scsi drives for reasonable money, so no problem here.
But all this is not the point. I wrote "desktop performance" by which i mean some GUI related ops. Not the speed at which Vista boots (on my 7200 Seagate it's up and running in about 50 seconds, no AHCI) or loads other software. Simple example is start menu, it's just sooo lagging. But i almost got used to it. And no, it's not my lack of "3 ghz" cpu, 'cos aero is almost completely rendered through d3d on a GPU which in my case is 8800GTS.
So..
It is not the HW that is my problem, it is my OS. Remember this: my hardware is as usable as the software i use, not the reverse. Without software, my hardware would be just a pile of junk.
And btw, when doing your DDR3 experience, don't forget to do real-life benchs, not synthetic RAM benchs, these will not prove anything. And real-life performance is what counts at the end.
Live long and prosper \/
P.S. and really remember: Modesty will bring you wherever you want...