The mini does have an available 2.66 as an upgrade option, although at $150, you're not really getting your money's worth.
The size of a monitor is really irrelevant. It has to do with the resolution of the display. The current mini can output up to 2560x1600 on a single display or 1920x1200 on dual displays. Don't see this as any sort of issue at all.
As for the CPUs, there is more happening there than just clock speed. There are a lot more strategies in the production of chips and what they're capable of than just clock speed today. The Passmark - CPU mark puts the chips in question in different realms. (Few people realize that there are maybe a half dozen chips Intel puts out with the exact same clock speed - they are not all created equal - this is one area where those that compare other off-the-shelf manufacturers price point fail to take into account - time and again I've seen Apple using the best chip in a particular clock speed, while many other manufacturers use the worst one to save a few bucks since the general public is not aware of this difference.)
The Benchmark for the specific chips in question:
1.83 - 1,033
2.53 - 1,803
2.66 - ranges between 1,903 and 1,973 - there are 3 chips at this clock speed rated above the 2.53 and with a quick search I can't determine which one Apple is using. Best case though is probably a 3-5% improvement over the 2.53 - once again, not worth the $150.
Bottom line, I believe you would see a noticeable improvement between your MB and the Mac Mini related to the CPU - With an 80% improvement in the benchmarks, I'd go so far as to say a minimum of 30-35% across the board and up to 50-60% on some CPU intensive tasks such as video encoding.
Couple that with the higher bus speed of the RAM on top of moving from 1GB to 4GB and the 9400M with 256MB standard vs the Intel GMA 950 with 64MB in your MB... (While the 9400M may not be a gamers card, it will just downright blow the doors off that Intel chip you have now.)
Upgrade the drive to a 7200RPM also...
While the Mac Mini is certainly no top of the line machine by todays standards, I believe you would be pleasantly surprised at the real world difference when running memory and/or CPU intensive tasks between the two machines you're talking about. imho... The current Mac Mini is a vastly superior machine when compared to your MB.
As dtravis pointed out, if you have the money for an iMac, I would go for one of those over the mini. But, I think you'd be happy with either one.