1) Are Pelosi and other such Congresscritters on both sides of the aisle overstepping the separation of powers?Um, NO, not in the least, not even a little bit, and comparing it to GW is, well silly. We are not talking about spying on US citizens here, from the article you provided, bold part done by me:
were in the Iraqi capital Friday at the head of a delegation of House members on a
fact-finding mission as the White House and Congress dueled over the expansion of U.S. forces in Iraq.
I will go a step farther- she was doing her job making sure one of the worst presidents in US history hasn't either A) Lied

been to stupid to breath (again) C) Mischaracterized the issues.
Pelosi has a DUTY to go to Iraq and investigate the presidents actions in Iraq, as regards to the "power of the purse" as a part of a check and balance against Executive branch's attempt to form a dictatorship.
ANY lawmaker can go on a "fact finding mission" when it comes to what they are spending our money on- in fact, has a duty to do so. This has been, on occasion, very dangerous for the lawmaker- anyone remember Jim Jones cult, and the lawmaker that died on THAT "fact finding" mission?
2) Is Pelosi treading upon Senatorial territory?Not even close- now, had she sought to usurp executive power and order the troops out- we would have a problem- but simply visiting there and going on a fact finding mission is just fine, and her duty as a lawmaker.
3) Should the Constitution be amended to reflect this practice?Looks like the only place the constitution needs to be amended in towards the executive branch in the following areas:
1) Can not commit troops for more than 10 days in any action, without a 2/3 majority vote from both houses
2) Can go to jail for not disclosing ALL evidence to lawmakers from both houses dealing with the mobilizing of troops
3) Anything sustained more than a year should have a national vote, 2/3 majority, every year troops are mobilized.
4) May NEVER override a judicial request for documents, notes or anything, ever, when subpoened, under pain of death for any cabinet member level and above that does not comply within 24 hours. Shredded documents carry the same penalty.
MORE judicial and legislative checks are needed for executive power- NOT less.