I don't think Dhoni will make a better opener than Sehwag. All sehwag lacks is foot movement, he is technically quite sound and has the require temperament. He does gives his wicket away but over the last year or so, he has improved very much in test cricket.
YOu cannot judge him by his ODI innings, thats completely misleading. He has played ODI like that for the past 4 years but his test cricket has changed, and thats one reason why he is one of the leading test batsmen today.
But sending Dhoni to open might click in this match. If he can make a cameo innings against the pacemen and then play it safe against murali as much as he can. We need Gambhir to click. This will relieve some pressure of the middle order. Sending Laxman 1 down is a better option.
Remember his innings against Aussies at Mumbai. I hope Dravid makes such move again. Dravid, like Laxman, can play with the tail. Even the tail made some effort yesterday, especially kumble and Agarkar. Every run is vital in the second innings and they better make Kumble count!
If India can run through SriLankan tail ASAP, then India would have the advantage.
From SriLankan POV, they will be thinking about keeping the deficit as low as possible or may be even gaining a slender lead!
PS: Sri Lanka will be expecting surprises from India. Either Pathan or Dhoni will be sent up the order. What we need to do is plan everything, making sure that every move clicks.
Btw, you can check innings highlights from Sify
Check this
Link