The Q8300 yorkfield will not work the only quads that will work is the Q6xxx which you will only find used but they do go for around $100 and are fairly good processors and will be a giant leap in performance compared to the 4300.
The E6600 wolfdale will not work the older E6600 conroes will. The wolfdales are 45nm and your board will not support it.
The L2 cache means nothing in the compatibility aspect.
No 1333mhz processor will work in that board nor will 45 nm processors work.
If you notice on there support page the E6600 listed is the 2.4ghz model which is the old core 2 duo conroe.http://processormatch.intel.com/COMPDB/SearchResult.aspx?Boardname=d975xbx2
So my suggestion is get the Q6600 off ebay or get a new board and move into an I5 or I7