I'm not sure I fully understand your question. The E8500 has 6MB of L3 cache, so no problem there. Why would you buy another mobo?
The E8500 is a very quick CPU and should be perfect for your needs. However, I am of the opinion that an SLI setup is overkill if you're only going to stick to 1280x1024. It does have some future-proofing though.
Apologies, I did some digging, and in fact it's L2 cache, not L3. Regardless, the E8500 is a great gaming processor. My brother has one and plays Crysis flawlessly at 1280x1024 paired with a single GTX260.