The market has changed drastically, in that it no longer changes nearly as fast. If that makes any sense.
Hardware is staying relevant and useful for much longer than it used to...and I only just recently replaced the computer that I built during Vanguard's friends and family beta.
In terms of i3 vs. i5... I think the i5 provides a great deal more flexibility and power. The graphics card is still of primary importance to games...but at this point it's a good idea to provide games with either more cores or higher clock speed depending on how they are programmed.
There actually are quite a few games which will utilize more cores, and more games like that are on the way... but for those games that don't, the i5 provides their up-clocking feature which boosts the clock speed of two cores when the other two aren't seeing use.
The hyper-threading in the dual-core i3 processors is not nearly as effective in a wide range of applications as legitimately having 4 physical cores. I think the current i5-2500K is probably pretty much the best price-performance CPU with a bit of a leaning towards performance, and it's been that way for quite a while now. It's pretty easy to put together a mid-range (around $1000) gaming PC with that CPU and a whole bunch of RAM.
For video cards, what price range are you looking at? What games are you looking to build the new machine for?