My personal opinion is that if you go with a C2D or C2Q system, you'll be able to have a really good system for less than a i7 system. This doesn't make it better, but the $ can be saved for other parts down the road. Here are my thoughts on a current gaming system:
* I'm not a big believer in the DDR3 situation right now (it's getting better, but not just yet). For good performing DDR3 right now it's still quite a bit more $, compared to DDR2. Yes DDR2 will not be mainstream for too much longer, but it's really cheap right now and with at least 4 gb's of DDR2 800mHz, it should last for quite a while.
* i7 mobo's right now are just too spendy for my likings (>$200). A good p45 based mobo ~$125 or so is plenty to last you for years to come, IMHO. Yes the i7 mobo should hold up for years too, just don't know if they will come out with a newer chipset that will give you more options and features that you might want anyway. This may or may not work out in your favor.
* I still think 2 core CPU's will still be the norm for 95% of games for the next year or 2. Yes there are some games that take advantage of the extra cores, but for the price premium of >$100 (compared to the new i7's), I'm not sure it's worth it or not.
* The current $300 or more price difference between a C2D/C2Q system over an i7 system isn't worth it to me at this point. You may see the $300 as an investiment in the future, but the future might change and you might want to change your mobo anyway for whatever reason. I'm still gaming well on a AMD Athalon x2 s939 system and don't really intend on upgrading until sometime next year or so. I don't play the latest on the highest settings, but I can still do whatever I want and don't really need to have the latest stuff out. Do I want the latest stuff? Yes, but financially I can't do it right now. This may be different in your case, so I thought I'd let you know where I'm coming from.
Here is how I see the comparisons (money wise):
i7 system:
CPU - $300 or more
Mobo - Starts at $220 right now
DDR3 - El cheapo RAM starts around $115 or so for 4 gb sets
Total i7 system (before adding in the rest of the parts) = ~$630
C2D/C2Q - $83 for e5200 (still can easily OC to 3.2-3.8 gHz) just add a $30 CPU HSF.
$120-190 for C2D's (e7200, e8400, e8500) or a q6600 (good choice still, OC's well too)
Mobo - $100-140 for a quality standard or above average p45 based mobo!
DDR2 - $30-50 for most quality DDR2 800 mHz RAM (4 gb sets)
Total for C2D/C2Q system (before adding in other parts) ~$210-$380 or so, depending on which configuration you go with.
Difference between the 2, before adding in the other parts (which will probably be about the same) = ~$250-480 or so. This difference is still quite large, AFAIC. This still may not be an issue to you, but I thought I'd throw out the basics between the 2 different systems (as far as costs go).
Is the $250-480 cost difference make a difference to you? I don't know, but for me, I'd rather spend the difference on upgrading any and all parts, especially the GPU, since this will be a "Gaming" system.
To answer you last question: Which C2D would allow you to play games for quite some time? I'd have to say the e8400 or e8500. They have a bit more cache and are already clocked pretty well, so if you start to feel that your system is lagging a bit, you can then just OC them up to 4 gHz or more for little to no cost to you! The e5200 (little brother of the e8400/e8500) is quite a nice chip that can OC to about 4 gHz too, but might not be the best choice for longevity. You could always upgrade the CPU (if you got the e5200 now) to a C2Q, when they come down in price within the next 6-18 months or so, but that would be something that you might consider ahead of time.