It sounds like you're trying to sell your management/boss on some idea. It also sounds like you're not a technical sales type of guy. If the "M" in "LAMP" stood for "Oracle" or "DB2", you could get Oracle or IBM to send out a technical sales critter to convince your boss. ;-) These are the guys whose livelihood is based on producing/selling, among other related things, highly available, highly performing database systems.

The question is, under a reasonable load, how many CPU cycles do you have left? For example, if your average is 10 hits per second, and your peak is 200 hits per second, check what the response times are for these scenarios. Check where most of your performance degredation is: is it in your database or in your CGI scripting? If it's the CGI, then what you want is multiple web servers. If it's the database, then you want a cluster of database servers.

Regardless of whether you stick with MySQL, or you switch to Oracle or DB2, you'll want an experienced professional to babysit your system(s). Or at least to set it up and tweak it, although doing proper backups is pretty key here, too, and that's an ongoing duty. That experienced professional need not be certified, but does need some sort of experience in whatever scale your site is (100MB, 1GB, 10GB, 100GB, 1TB - how much data do you have? And reliability - how many 9's do you need?).

There are just too many variables to say that it would be "better" (what is "better"?) by a factor of X. I suppose I'd start with whether your system is fully (or nearly fully) loaded as it is. If you have disk I/O and CPU cycles to spare, then splitting the two tasks will do nothing for speed. Security, yes, but speed, no.