Here is a nice piece of advice in this matter. No one else would give you.
I have observed, that once you buy a good card, a high end one, then generally you would not need to upgrade the card for at least two years, and the card may last up to even seven or eight years (unless you have an unreasonable itch for the new things). Five years-- is my guarantee.
But here, you want that the whole PC should last 5 years. That may be possible. Though it would seem to one that 5 years is just too much, remember, that upgradation after 2-3 years is for the ones who buy low end systems; not for those who buy the higher end systems.
Let us see what configuration comes out as the best one-- these are just the main components:
1. You should buy two Sapphire ATI Radeons 5890s (Trust me, even a single 5870 is enough for 5 years and for a display size of 56 inches! Yes, for all the future games for at least 5 years!)
2. The processor, if you can afford, then the Shanghai one from AMD must be it. Which one among them, depends upon your budget. But you cannot compromise on the processor speed. You would need to buy a good mo'bo' and one of the fastest high-end processors available today. It could be an iCore from Intel, but be sure that it should be the one equivalent to the "Extreme" quad core category by Intel (like QX6850 etc).
3. The PSU should be the rather recent launch from the Sapphire Tech, powered 1250 watts.
4. Buy the fastest RAMs (ask for the RAM speed), like they have DDR5. But if you don't get it, then 8GB of DDR3 (1866 or 1600 mhz) should be just more than enough for 5 years.
5. Lastly, buy an SCSI hard disk with the highest speed.
True, the above may exceed your budget, but there are 2 things to consider. One, the margin in the above configuration is tremendous. So you can make cuts in the configuration to fit it in your budget.
Secondly, if you don't buy the above configuration from a branded company, instead get it assembled from a good local computer vendor, then you sure could get a much better deal in the above configuration. If the PC is properly assembled, then trust me, in 5 years, not even once should you need to call your computer guy. (Ensure you have earthing where you use the computer.)
I think if you stick to the above configuration even for 5 years, including the processor, then all that would happen with the future games is that the frame rate would decrease, but very slightly. I don't think the difference between your system and the contemporary system after 5 years would be enough for your eyes to even notice! Yep! And whatever games your PC won't be able to play then, the contemporary PCs wouldn't handle at all, either!!!
Enjoy!!!