Gigastudio is standard for orchestral scoring inside a DAW along with Garritan libraries which can be used with many samplers. It's in heavy use in the industry. I'd also recommend Sampletank, you can get the free version with some very nice samples included.
NI also have a very nice Orchestral & String Kontakt instrument. These are all fairly pricey though, but the quality is amazing. Look to Big Fish Audio for orchestral samples as decent prices, many multisampled instruments as well. Multisamples are pretty essential.
I've done some scoring work, and Nuendo as well as Sound Forge were invaluable for bigger projects. Nuendo is basically Cubase with some special features, so if you're familiar with that...
A good MIDI controller is also a must, including a wheel to skip timelines frame by frame, or at least a few knobs. Any good NLE video software will also accept MIDI controllers and VSTs.
Not used Ableton Live for it, though I'm sure it's possible. Can you import videos into a timeline? This is essential.
FL Studio XL allows importing of many video formats and will play them in sync. I've done quick scoring jobs in there for people's smaller projects (30 second ads etc), and it was sufficient, though for surround mixing it's not ideal without 3rd party tools.
You will find effects don't help make them sound more realistic 99% of the time, aside from reverb, in which case you'll want convolution types and good impulses. Search the forum, I posted a bunch of links for that stuff in the last month or so.
It's all about how you play/program the sounds. Do NOT process the sounds much, do NOT EQ much at all. You generally want to stay away from dynamics processing if you want them to sound live, as in no compression, or very light if any.
For a look at it from an electronic/dance perspective, take a look at Sabre's masterclass. He does an orchestral inspired tune with Garritan and has some good tips:
http://www.vimeo.com/m/21570255