I've read all the benchmarks about the new processors and can understand all the hype around the new C2D.
I've been considering replacing my system and have been finding it difficult to make the right choice, socket 775, AM2 + DDR2, socket 939 etc
I don't feel that my Rig is past its sell by date quite yet, I'm not afraid of overclocking it when I need to, my first build a 2000 T'bred & 9500 pro showed me the rewards of doing it and taught me a lot.
Nowadays overclocking my machine isn't giving me the sudden boost to frame rates it once did.....understandibly. As my motherboard is an old 478 socket and old AGP when i come to upgrade it'll be from scratch....no migration of componets here.
I've got the budget but I don't think it's worth it just yet what with Directx10 showing on the horizon I reckon my machine can make into next year and wait until the prices fall a little, so is it worth spending £100 litterally £100 on a Radeon X1600 512 to give it an extra boost that'll last long enough for it to be justified stop gap. Playing HL2 - EP1 nails my machine it's actually surprising just how much my machine slows, guess the Radeon 9800 has aged badly.
I think buying a 7800 for £246 is false economy I mean that's 6600 and DDR2 right there.
I can't tell you the amount of times I've had all the items in my basket and about to purchase them all before backing out but when i do upgrade I don't want it to suddenly become an out of date technolgy, for instance my Socket A pc's and Socket 478 have seen various CPUs on them and have lasted a long while and are still happy as storage servers and Linux installs. Sorry about the long post but there's so many options for me and I'm not 100% sure I'm making the right decisions.
I always say not to wait 6 months for some new technology "just on the horizon." For when the first DX10 cards are released they will say "in 4 months the DX10 cards are going to put the first round of DX10's to shame" Smartest thing to do is leave yourself with an open upgrade path.
939 is a dead platform, won't be anything new coming out for the platform.
AM2 is very new and will be a good purchase right now.
775 is C2D obviously, good purchase also.
DDR2 all I can say is you should invest now in your memory, prices are constantly rising slowly on DDR2 now that AM2 requires it and the demand is increasing.
In short, upgrade now to what you want, leaving yourself with a wide open upgrade path and you will be very happy.
Look into kentsfield compatibility for current C2D mobos and get you a e6300 that'll beat the pants off most everything out there with a mild overclock and you should have a decent upgrade path for a good while as well. I admit I'm waiting on the next batch of C2D compatible boards from nvidia and such to make my purchase but thats as long as I'll wait. Soon as they hit I'll do just what I'm suggesting. If I were you I wouldn't wait longer than the next boards arriving so you have a better choice of mobo. Think they were supposed to be out this month but maybe as late as october. I have a very similar build to yours and it will do for another month.
I agree with the other poster in that you should never wait too long to upgrade however cause theres always something new coming out. If you really want an upgrade and have the funds do it soon. My only side-advice would be maybe getting a good midrange graphics card if you build now and be prepared to buy a next gen card when they arrive, thats my plan at least. Good luck whatever your choice!