Okay,
So many questions answered here, but this forum stinks to high heaven to gather them in one location. Is there such thing as a 'sticky' here?
I can get to Chengdu, but from there to Lhasa, what can be paid with credit cards vs cash? How much cash is needed? How long of a wait to get train from Chengdu? How long of a wait to get plane?
These are all questions, although may not be worded as such:
1) Hostel in Chengdu I assume are cash.
2) Hotels in Chengdu are cash or credit, I assume.
3) Once in Chengdu, how much time is needed in advance to purchase train tickets to Lhasa? I've heard everything from 2-3 days to 2 weeks.
4) How much is a train ticket? hard vs soft sleeper? Can it be paid with credit or only cash?
5) Once in Chengdu, how much time is needed in advance to purchase plane tickets to Lhasa?
6) How much is a plane ticket? Can it be paid with credit or only cash?
6a) How much would a woodchuck chuck if a woodchuck could chuck wood?
7) Is it possible to hire a jeep from Chengdu to Lhasa? How much (this will be cash I'm sure)?
8) Once in Lhasa, how much time in advance is needed to purchase plane tickets to Chengdu? Credit, or only cash accepted?
9) Once in Lhasa, 2 days to aclimate, go to local tourist group (can't remember name) for a 4-5 day jeep tour. Cash only again?
10) How much total cash, not including food/hostel/hotel?

I think cash is god in China, most budget places won't let you use credit card.
1) Hostel in Chengdu I assume are cash.
Yes.
2) Hotels in Chengdu are cash or credit, I assume.
Yes.
3) Once in Chengdu, how much time is needed in advance to purchase train tickets to Lhasa? I've heard everything from 2-3 days to 2 weeks.
Better be 5-6 days. Train can be booked only 6 days in advance.
4) How much is a train ticket? hard vs soft sleeper? Can it be paid with credit or only cash?
Cash only, last year it was Y712 for the hard sleeper, should be around Y900 for a soft one.
5) Once in Chengdu, how much time is needed in advance to purchase plane tickets to Lhasa?
Even one day in advance, flight ticket is usually available, but the earlier you book it, the cheaper (maybe).
6) How much is a plane ticket? Can it be paid with credit or only cash?
Around Y1500, sometimes you may be able to get cheaper tickets, especially from Chongqing.
6a) How much would a woodchuck chuck if a woodchuck could chuck wood?
?????
8) Once in Lhasa, how much time in advance is needed to purchase plane tickets to Chengdu? Credit, or only cash accepted?
Cash also, at least a day in advance.
9) Once in Lhasa, 2 days to aclimate, go to local tourist group (can't remember name) for a 4-5 day jeep tour. Cash only again?
Yes.
10) How much total cash, not including food/hostel/hotel?
I guess at least Y5500. If you're flying in and out of Lhasa, join a jeep tour to EBC (around 4 to 5 days).

I just got back from China (including Chengdu, but not Tibet). Bottom line is, except for high-end hotels, virtually nobody accepts credit cards...not even most "upscale" restaurants, unless they are affiliated with an upscale hotel. I found that I was constantly in search of an ATM that accepted "Western" cards. China Construction Bank usually worked for me.
That's about all the info I can provide based on the questions you asked. Good luck!

My flight one way fom Chengdu incl. TTB cost Y1750 I booked through my hostel and left the next day. Bear in mind this was in 2002 and the price will almost certainly have gone up. The train might mean a longer wait.
If you can spend a bit longer in Lhasa to acclimatise it is advisable. Tw days will probably be enough but more is better.
WeiLong in Tibet

We paid about 1500Y for soft sleeper including permit from Chengdu to Lhasa. Bear in mind tickets can be hard to get and you may have to take whats available. This was in April just gone and before the travel problems. We still didnt know about the tickets till 3 days before travel.

to #3, flight prices certainly have gone DOWN after the train was introduced in July 2006
for current prices you can check at www.yoee.com or www.elong.net/flights
Agree with #5. Last October, I paid 2500Y for a round trip airline ticket (Chengdu - Lhasa - Chengdu). The price for a one way was listed as 1300Y. That included the Tibet permit. That was through Sim's Cozy Guesthouse in Chengdu.
To the OP -
#5 & 6 - Most places can get them for you in a couple of days, if there are empty seats. You can also make arrangements in advance, and get you ticket within a day of arrival in Chnegdu. I am not sure if the current travel restrictions have affected this.
#6a - It is currently against the law in China for any woodchuck to chuck wood.
#9 - I would suggest going to the FIT office at the Banak Shol hotel to arrange travel. Yes, it is cash only. You can post notices at the various guesthouses in order to find fellow travelers, and form your group.