Successfully installed BTS .. and I think BUG MOD 1.0 too .. (But didnt know in which folder it should have been installed .. It installed by default in 'documents/my games/BTS' folder probably)
The current BUG mod version is 4.4. The proper location for it, usually, is C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Beyond the Sword\Mods. You should then be able to load it by either (a) starting BtS, selecting "Advanced" then "Load a Mod". BUG (or whatever the folder under Mods containing it is named) should be in the list of mods you can select; or (b) create a BtS shortcut (easiest way--copy the existing one in your Start menu) and modify the Target by adding
to the end, again where BUG is the name of the folder under Mods where you placed the BUG files.
* Anywy I am right now being able to play BTS 'only', CIV iv original and Warlord expansion not working, may be some problem with my installer

.. Anybody can help me with a suitable torrent for BTS? ..
No one here will help you download Civ or any of its expansions illegally. Pay for what you play so we keep getting future versions of Civ (hopefully better ones than Civ V).
Vanilla Civ IV and Warlords should both still work. BtS doesn't stop them from working. Just find the right executable (or shortcut to it) and fire it up.
* Dnt know wheather the BUG MOD installed properly or not, but can see the turns as "5/750", but not as "Turn - 5" as I hv seen in some of the screenshots ! Is that due to a different mod?
If you are indeed running BUG you will see additional progress bars at the top of the screen: one to the left of your research bar for your Great General progress, another to the right of the research bar showing your Great Person progress. You'll also see a BUG options button at top left right next to the Event Log button.
* I hv seen normal game is now of 500 turns. But earlier in CIV iv it was of 460 turns probably. Does this change in BTS only? In BTS epic game is of 750 turns. How much was it in CIV Vanilla?
AFAIK the number of turns is consistent across all versions of Civ IV: Quick=330, Normal=500, Epic=750, Marathon=1500.
* It looks in chieftain it has become a bit tougher to play the comp opponents in BTS (I know its funny, but I hv played only one game of CIV so far, so even chieftain is tough for me

) .. Is the BTS AI are tougher than the vanilla?
Phew !..
Yes, as I recall the programmers at Firaxis beefed up the AI in each expansion pack. Once you get better at the game you can try my favourite mod, Better BUG AI, which combines BUG with the Better AI mod, further enhancing the AI. It's still not as smart as a human opponent, but it does keep the AI from many stupid things that annoyed me even on the lower difficulty levels.