You're pretty much in the same boat as me - I also still use an EX58/1336 base.
For all systems, the limiting factor is always the motherboard chipset (and accompanying peripheral attachment!) lifespan.
I purchased a PCIe sata III card. This allows me to attach a second SSD and further HDD (both sata III). However, my OS is installed on a 256GB Revodrive X2 PCIe SSD. My other specifications can be seen.
What I'm trying to say is that 3rd party cards can bridge some gaps in order to prolong any base's (i.e. motherboard) lifespan, and additional options (such as PCIe SSD's) can be explored.
Overclocking, of course, also helps.
Practically all motherboards allow the primary components to be upgraded (memory,gfx card/s,cpu) - unless they are already maxed.
What you need to do is to find out if there is anything - significant - you can gain in upgrading your current system.
What you - and I (and everyone else) - will eventually need to do is to assess our current systems and upgradeability in balance with the current and emerging technologies.
--> Do you need to upgrade/replace now or can you wait longer for more 'significant' technological benefits!
Always a difficult and costly choice, but if you are still happy now then there is no reason not to continue waiting.....a little longer
Edit: your first port of call should be an SSD (120 to 240GB (ideal)) for your OS and primary applications/games. An SSD makes a huge difference to loading and general desktop. responsiveness. Chose wisely since it's also transferable to any new system.