Re: CMOS Check Sum Bad
If the Memory page says 400MHz that means it's running at 800 double data rate which is still slow for your computer. As AR said the RAM probably needs to be set to 1066MHz manually then re-timed. Try just setting your bios to default. If that doesn't work then put in the timings manually for your RAM.
Should be PC2-8500 RAM. I believe the 750i's are supposed to run at 800MHz stock for the memory bus but can be "overclocked" to 1066. If your RAM is rated for 1066 but your board is rated for 800, going to 1066 will be considered "overclocking" and you have to do it yourself. Just set your DRAM:FSB ratio to 1:1.
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