Ant, you've got a pretty decent system there to begin with - what caught my attention is that you say you have near 100% CPU utilization during busy times. I can't imagine the apps you mention using taking that much horsepower unless there's a serious bottleneck somewhere.
Work on that utilization and I bet you'll be much happier.
Do thorough virus/trojan/spyware checks; I use AVG antivirus and both Spybot and AdAware spyware checkers (each finds stuff the other doesn't) - all these are free for personal use. Run a disk-defragging program (although with those large drives it may not make much difference).
If you really have 400-500 Mb memory left over adding more probably won't make much difference.
You might look into disk I/O as well; if you're using slow drives and/or controllers you CPU might be tied up either doing transfers or waiting for them to happen. MetaStock and similar apps spend a lot of time moving relatively-small bits of data back and forth between the Internet, memory and hard drives.
Go here
http://www.sisoftware.net/index.html?dir=dload&location=sware_dl_3264&langx=en&a= and download Sandra Lite for free - its benchmarks may give you some clues about where things can be improved.
Note that big price drops from Intel and AMD are coming later this month on their high-end chips and new dual-core models are due by the end of the year - all this means that system price reductions are coming as well; you can get the best value by buying processors that are 2-4 notches below the top of the line.
For your needs (and mine), putting the money into CPU throughput and disk I/O is more effective than total memory and graphics, which is where the gamers spend their money.
If it were me, unless I was ready to throw the computer out the window, I'd live with it until Jan or Feb next year; there are always great deals on the best systems in the post-Christmas doldrums. Just like cars.
And one more thing: if Microsoft meets their deadline and rolls out Vista early next year, there will be *many* deals on high-end systems that don't quite meet full Vista specs. That should be ok, because I'd recommend sitting on the sidelines for a year or so at least, to watch others' experiences with Vista before making the move. And maybe not even then; waiting for Service Pack 1 or 2 seems to be a good idea for new OSs.
When you see that in order to produce, you need to obtain permission from men who produce nothing - when you see that money is flowing to those who deal, not in goods, but in favors - when you see that men get richer by graft and by pull than by work, and your laws don't protect you against them, but protect them against you - when you see corruption being rewarded and honesty becoming a self-sacrifice - you may know that your society is doomed. - Ayn Rand