If it is lead paint on the inside the remediation could be huge.In some cases if density levels have increased for zoning the answer might be to tear it down and build new.

What are some little "inexpensive" add-on / upgrades can I do to the home help increase the rent while at the same time giving them the security that I am trying to make this place better for them?

You need to concentrate on improving your employment situation and increasing your income.

So when rents move up from $500 per apt back in the year 2000 to around $1,900 today for one of my buildings, the value of the building increased from $140k back in 2000 to $1.4 Million today.NYC is a Future Value (FV) play.

@David Cruice Primarily because i like the cash flow on the property where it is so i don't want to increase the monthly payment or see a rate increase. 

You'd be better off partnering with one or two other people in a deal this size to increase returns for everyone.

When the cost of money rise (higher interest rates) cap rates increase. its simple investment fundamentals.  

Or would a decent sized house attract more renters.Any advice would be greatly appreciated.

What attracts the best tenants?

As for learning how to increase your current cash situation you should listen to the BP Money podcast.