so i must go FF , with tse -17 and 24 .... . sorry but one more question : do you think having 16-35 mk2 or24-70 is at all nessecery along my 2 tse lenses , or a waste of money ? and one more thing .... my power shot is veryweak on iso performance ( max 400) so most of the time i need a stable surface as a tripod ( because i dont have one ) specially for interiors . so after all this ... do you recommed 5d2( my iso problem) ?
For architecture it's not necessary to have a 16-35 zoom if you have 17 & 24 TSEs. But, you may want to have the short zoom for other reasons - maybe for sports, kids, hiking. So, if you don't want to spend so much money on lenses you might choose to get a 16-35 or a 17-40 instead of a 17TSE if you have other uses for the zoom. Then occasionally when the 24TSE isn't wide enough you can use the wide end zoom for architecture and just be careful to not tilt the camera too much or do minimal perspective correction in post process.
The EOS 5Ds are good for architecture. The other (more expensive) EOS full frame bodies are geared more for high volume, speed/action shooting - so if you don't need the speed for other reasons there's no need to spend the extra money on other EOS full frames. For architecture, I don't think the 5Dmk3 is necessary, 5Dmk2 is fine.
But, here are some benefits of 5Dmk3 over mk2:
mk3 has up to 7 frame Auto Bracketing, mk2 has only 3
mk3 does +-5 EV Auto Bracketing, mk2 only +-2
mk3 can use CF or SD/SDHC/SDXC cards, mk2 only uses CF cards
mk3 has 61 AF point sensors, mk2 has 9 points
mk3 shoots continuous 6 fps, mk2 shoots 3.9 fps
The extra auto bracketing on mk3 is cool if you do a ton of HDR. But, it's not too hard to just bracket manually - shoot, dial 3 clicks, shoot, dial 3 clicks, shoot, dial 3 clicks, etc. The extra AF points and fast fps are not necessary for architecture.
If you're going to shoot architecture with a shift lens you have to use a tripod. The shift setting is not going to be accurate while hand-holding. And, often in architecture you want a lot of depth of field so, you use a tight aperture and slow shutter. Also, nice to shoot long shutters to keep the building sharp while letting trees, water, people blur out.
Yeah, the newer DSLRs will have way better ISO, less noise than the small point&shoot cameras. But again, you'll probably want to shoot architecture in mid 100-400 ISO and need a tripod anyway.