Comparing iPhone 4 and iPhone 3GS
Apple has made this table to compare the features of the iPhone 4 with the previous iPhone 3GS. Please note that these prices are with the US carrier AT&T and is tied to a 2-year contract.
When Mobily get the iPhone 4 in Saudi Arabia the price will most likely be the same as the previous generation prices.
Arabic version of this post on SaudiMac Arabic Edition.