I have a ~4 year old machine. I have 80 mods installed, including all DLCs, FOOK, Project Nedava, EVE, Nevada Skies, AWOP, Arwen's, and even WARZONES (although warzones really strain my aging machine).
You might want to avoid mods that are known to be performance heavy (like high-rez textures which need lots of memory, or WARZONES which adds hundreds of battling NPC... it's a CPU hog), but otherwise there's no "computer strength" limit.
For storage and display, I recommend http://newvegas.nexusmods.com/downloads/file.php?id=37884 .