We don't have any hard data yet on the i5, although this article does show that for most uses, the i5 represents better value for money, if prices on release are as expected.
http://www.hardocp.com/article/2009/...mance_preview/
Better performance depends on which chip you buy. The i5-750 looks like a great deal, but lacks hyper-threading (think 955). Given that few applications make full use of HT ATM this may be of little value, and would be a smart choice. The i5 8-series will have HT and similar levels of performance to the i7 920 or 940, but will cost more
OTOH - the i7 920 and the 1366 mainboard will allow eventual upgrade to the i9. Prices are likely to fall on this good value chip. (I'm reminded of the old 300A back in the day) It really depends on your usage and your pocket.