In almost all games (and at the same clockspeed) i5 (700 series) and i7 CPUs are almost exactly as fast as each other. In fact the i5 is sometimes faster due to the way the PCIe bus is connected.
The main benefit of going for an i7 is that it has hyperthreading enabled, however hardly any current games make use of this. In the future hyperthreading may be better exploited in games, but at the minute four processor threads (or even two) is usually enough.
The other main benefit of the i7 900 series is the use of X58 boards. These boards almost all allow SLI and Crossfire, also they can run a dual-GPU system at full PCIe v2 x16x16 speed (the i5's P55 boards are limited to x8x8 speed). However, if you only plan on using a single card then the P55/i5 option makes much more sense.
Another good option is an AMD Phenom II X4 system, this will cost even less than an i5 system and in games it will be just as good.
Also, just so you are aware, AMD and Intel are releasing brand-new CPU and motherboard generations at the start of next year (Q1 2011). For gaming this won't give you much extra performance (as games performance is almost always bottlenecked by graphics power) but if you have a decent current system and don't need to upgrade right now - then waiting would make sense. May I ask what your current rig is?
you never know wots round the corner that could use this power
