I am in exactly the same boat Howie. I'm pretty tight on funds, need a new gaming rig, and i want a Rig that will last as long as it can (i.e. one that is a relative monster right now so that it will take a while to become obsolete).
My projected specs right now are something along the lines of AMD Phenom quad 2.2, Asus hybrid crossfire M3A-H, etc. I'm trying hard not to spend over 1k dollars and am only building a tower. So, regarding your question about the video card conundrum, I hear ya all the way. The prices are so sweet right now and I've been stalling as long as possible to see if prices drop further. Here's my 2 cents after wasting many days researching the cards:
In terms of investing long term, I'm thinking the HD4870 is probably best. I was very close to just ordering a 4850 to save the cash but in the long term, you're probably saving more by the 4870 investment, especially considering the 4870 X2 should be available by mid to late August (only about a month).
With that said, however, I'm probably going to wait a bit longer because I wouldn't be surprised if the price on both the 4870 and 4850 continues to drop in anticipation / realization of the 4870 X2 release. I can see a month from now regretting having made that purchase when looking at the lower prices.
I am rather inexperienced with the hardware market though, I've only built 1 Rig in the past, so it would be really helpful for others to comment on this please!
All that said: I would go with HD4870.