The Microcenter prices are not sale prices, but rather are available all the time. The current prices are: i750 for $150; i920 for $200; and i860 for $230. Don't worry about these prices going up; they have remained at this level or gone down during the last two months.
Question 1: "Is the i7 920 going to be substantially better than the i5 750 in regards to gaming, dvd burning apps, etc?"
The conventional wisdom is that the i920 / X58 system is the fastest mainstream computer available today. That was true back in August, but that position is being challenged with the introduction of the i750 & i860 / P55 systems. Most people do not realize that the i750 / P55 system actually outperforms the i920 / X58 system when using only one graphics card:
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=364...
But most people realize that the i920 utilizes two PCIe X16 slots when two graphics cards are installed, whereas the i750 / P55 system is limited to two X8 slots in this situation. However, in actual performance testing the improvements are miniscule. In fact, in some cases with two graphics cards, the i750 actually outperforms the i920, and visa versa, but these systems are essentially identical:
http://www.tomshardware.com/reviews/core-i5-gaming,2403...
Question 2: "Would the overclocking potential of the i7 buy me a year or more of use over the i5's potential if overclocked?"
Both the X58 and P55 platforms have good overclocking potential. The following article shows the performance of the i920 / X58 system compared to the i860 / P55 system at stock speeds and overclocked to 4.2 GHz with either one or two-crossfired ATI 5870 graphics cards:
http://www.anandtech.com/mb/showdoc.aspx?i=3649&p=6
Once again, the performance of these two systems is nearly identical with the i860 outperforming the i920 in some applications, and visa versa.
One of the reasons the i750 & i860 / P55 platform does so well is because these processors have incorporated the PCIe bus onto the chip, thus eliminating bandwidth bottlenecks associated with the Northbridge chip required in the i920 system.
The bottom line is that they are all great systems. A typical i920 build (CPU + MB + 3X2GB RAM) will tend to be about $200 more than a typical i750 build (CPU + MB + 2X2GB RAM). For most gamers, this $200 would be better spent on getting either a better graphics card, or adding a second graphics card.