Alright, so I managed to get my first model done and rendered in a basic pose. But I would like to keep as much of the lighting detail as possible, as it helps shows off the look that I'm trying to achieve.

I was wandering about what would be a good HD resolution to use in an isometric game these days without worrying too much about blowing the video memory under reasonable use.

Here's an image that I've made with the help of blender to give a comparison.

http://imgur.com/a/nohHQ

The dimensions for this one was 256x384, where the flat tile resolution is 256x124

Well, assuming here that you are making a 2D game since you're worried about resolution? You can go ahead and easily make your tiles 256x128 which is plenty of detail. If you really needed to you could do 512x256 (although probably pointless).

Yeah it will be in 2D, but I'm also worried about the size of the memory requirement of so many sprite sheets ^-^; I don't know the video memory specification of the computers we will be developing for.

Make sure your code is set up to load and unload un-used resources along with not displaying resources that aren't on screen. Aim for about 1GB max of video memory. I'd say that's plenty.

At 256x128 resolution per tile the data per tile will be about 128KB or less which means you could have 100s of tiles with no problem. There is no need to worry about graphics memory with these types of games really.