Windows 7 RAM Installation - Memory Mismatch Error

I have 8 ram slots with one gig sticks in each which gives me 8 gigs of ram, Which my computer uses completely. I recently purchased 8 more gigs on two sticks, but when I install them it gives me a memory mismatch error, but it stills starts up. When I go to my computer properties menu it says 14 gigs ram (4 usable). Like I said there are 8 slots, 4 with white push tabs, 4 with black push tabs. I tried multiple combinations with the ram. Sometimes the computer would beep and not Start, others it would start up but still give me the mismatch errors. Need to update BIOS, Which I did. I have to change my BIOS settings to "Enable dynamic memory remap", but the problem is that I can't get into my BIOS. For some reason windows 7 isn't giving me a BIOS option (DEL) on the POST screen. It only gives me the option of going into system settings F2 or Boot menu F12. I tried ESC F1-F12 DEL END...to no avail.

System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Professional, Service Pack 1, 64 bit
Processor: Intel(R) Xeon(R) CPU E5420 @ 2.50GHz, Intel64 Family 6 Model 23 Stepping 10
Processor Count: 8
RAM: 8189 Mb
Graphics Card: NVIDIA GeForce GTX 285, 1024 Mb
Hard Drives: C: Total - 1907626 MB, Free - 1720199 MB; D: Total - 99 MB, Free - 71 MB; E: Total - 953766 MB, Free - 88455 MB; H: Total - 715202 MB, Free - 175942 MB; I: Total - 953603 MB, Free - 736122 MB;
Motherboard: Dell Inc., 0RW199
Antivirus: ESET Smart Security 5.0, Updated and Enabled