I've got a Toshiba Qosmio X505-Q890 that I bought back in 2009. Nvidia GeForce GTS 360M graphics, 6 GB of RAM, smallish SSD for the OS and a 500 GB HD for storage. Intel Core i7 CPU running at 1.73 GHz.

So what kind of performance should I expect from adding the mods I've got checked in http://www.gamesas.com/topic/1390399-my-list-of-mods-installed-so-far-advice-requested-from-those-in-the-know/, meaning specifically Beautiful Whiterun, More Dynamic Shadows, CLARALUX, SMIM, and Realistic Lighting with Customization? I've only played it at night briefly, but I didn't enter any interior cells, it was from a reload and not a fresh start, so I don't know how many of the mods "took" yet, I didn't tinker with Wrye Bash or BOSS yet at all, and there weren't a lot of lights around at the time, so I don't know how much of a test it represented.

I can't even find out how much the GTS 360M has in the way of VRAM, either. The data I've found via Google regarding the GTS 360M:

Memory Speed: 1800 MHz
Memory Bus Width: 128 Bit
Memory Type: DDR3, GDDR3, GDDR5
Max. Amount of Memory: 1024 MB
Shared Memory: no

So how much VRAM does that even tell me I have? 1 GB?

Should I expect decent performance from this 3 year old system still, given the mods I've added so far?

With a 128-bit memory interface, I wouldn't get your hopes up too much for consistency.