As far as TS3 is concerned (I never had the chance to play with TS1 or TS2 properly), computers seem to be mostly dominated by laptops and flatscreens. For the aspiring writer, there's also a rickety looking flatbed for your budget.
So why is it that the icon in buy mode shows a monitor and a tower?
The only time I've ever seen a tower in buy mode wasn't even in the computer section, it was a gutted out old thing that had been placed in miscellaneous decor to serve as clutter. Not that I wouldn't like to see more clutter like that (tools, art supplies and stuff), it took some time to build my sims a proper messy garage, next comes a proper messy workshop/toolshed.
Anyway, TS3 doesn't seem to have any towers, which is kind of disappointing for me because towers come in all shapes and sizes, and for all budgets. You can have a rickety old thing that puffs out dust every time you fire up the fans, or a superpowered gaming machine loaded to the brim with all the fixins' like LEDs and liquid cooling. Someone from MTS has even put together an Alienware look alike.