This is a tough question. Most of your parts are pretty good.
I'm not sure if you'll see much if any preformance increase going from 2 3870's to one 4870. If you went to a 4870x2 or two 4870's you would see an increase in preformance.
If you go with a higher end graphics card your 7200 might be on the verge of being a bottle neck. Depends on the situation but it could become a bottle neck later on. The question though is, should you go with the q9550? This is tough seeing you are doing updates on a regular basis vs waiting so long and basically making upgrading not a possiblity. (i applaud you on this). If you do the q9550 you'll start hitting a wall on future upgrades. This is because of the new i7 cpus. Its not easy to upgrade to this though as you'd need the new cpu, mb, and ram. That's a pretty penny.
If you need more space than grab a larger drive. the 640gb drives are pretty good drives. If you wanted you could pick up the 300gb raptor thats been on sale at newegg the last few days.
Do some research on windows 7 release date. Might want to jump right to that?
Oh yeah, Windows 7. I been reading about it. The new cpu graphics acceleration is nice if you ever needed it.
monitor might be a good upgrade. My current one is nice but refresh limits me. It is a samsung syncmaster wm22. 75hz max. 1000:1 contrast. A 24" wide with a better contrast ratio and refresh would be nice, maybe a HD monitor. Would that give me a better gaming experience?
No need for more storage now. I also have a 200G maxtor USB drive. that is a total of 950G. I barely have 100G used including backups.
the i7 issue is big, but that is why i want to get a c2q q9xxx in the spring. that will last me a year or 2 with new multi core games, in the mean time prices will come down on i7 and ddr3.
Looks like a monitor is in the lead at this point.
ya windows 7 looks like it might have some surprising features.
Probally won't notice to much of a difference with the new monitor being
HD, with your current GPUs. This would be something that would have to be upgrade in the future. I'm not totaly sold that you should upgrade here but there are some better options out there than the current one you have. If anything it'll give you a dual monitor setup if you want?
when i noted about the Hard drives it wasn't for storage capicity. It was for speed. A raptor runs at 10k rpm vs 7.2rpm. This will increase your boot and loads times for things.
If you plan on upgrading to the i7 in the near future i wouldn't do anything that deals with ram/mb/cpu. the i7 will need all these items replaced.
Honestly I think you've got a pretty good rig the way it is. Maybe add some speakers? *shrugs*