What I don't understand is you are asking for 'fast' PC but don't do anything heavy duty on it. You can load it with 6 (3 x 2gb modules) or 12GB (3 x 4gb modules) of RAM but you won't see much benefit. For general web browsing, e-mail, watching movies, etc you can easily do very quickly on a i5 based system for 1/2 the price.
But if you are really determined to spend your money then the single biggest difference you will notice is the SSD. As cheekymonkey mentioned boot times and application load will be VERY quick compared to HDD.
After that a gfx card with HD support as more software (like flash) will be using hardware HD acceleration in future.
I know you are trying to future proof for 5 years. Main items that you are likely to upgrade are,
Hard Drive - Go for a motherboard with SATAIII support.
CPU - Intel are quick good about changing sockets less frequently. I expect socket 1158 and 1366 to be around awhile. They will continue to release high end stuff on 1366 and mainstream on 1158. The only thing that might catch you out is if they bump up FSB speeds.
Memory - I expect DDR3 to be around a while the higher end motherboard that 6 modules giving you some options if you go with 3 x 2GB modules.
Graphics card. PCI-E is going to be around for a while. most of the top boards are now supporting two 16x slots leaving plenty of options for the future.
