I already bought Tropico Reloaded from Impulse,but i'm looking forward some really good old games too.Already picked up Evil Genius ,Disciples 2 Gold,both Stronghold gamess and Seven Kingdoms 2.
I prefer single player games and strategy.
Any suggestion?

Either Heroes of Might and Magic II (or wait a few weeks for HoMM III) or Settlers II...

Get The Guild. It is single player and has some elements of strategy but it is more of a life sim than an RTS.

Depending on games you bought you may like: Tha Nations (Alien Nations2), Port Royale 2, Settlers 2 Gold

Maybe i pick up Jagged Alliance 2 with expansions.

But be warned it's hard game, not RTS and tactic is really important.

I second that.

If you want an INTENSE CHALLENGE, get Soldiers: Heroes of WW2. Well worth the price and the such. Check some videos before you decide to buy it if you do.

Is Heroes of Might and Magic II and Settlers II using DOSBox?

Yes, both of them are using it.