I havn't actually done it yet but I'm replying here because it's been on my mind lately and is something that I plan to experiment with.
My vision is to have my serato set up on my Mbp and then another cheap small pc laptop running ableton with one small midi controller like my original x-session or something. The idea of running both on the same computer just seams sketchy to me. Plus, having the two laptops add some backup reassurance in case one of them crashes.
As far as using ableton as an advanced fx box I could only imagine doing it properly by running thru a small external mixer, using the send /returns. You could run straight in, but that would not only add a little extra latency but it could also potentially degrade the overall dry signal, which is of huge concern to me (Infact, I stopped using my Korg Kp3 in my previous setup (running direct in) because I noticed how much it actually degraded the un processed signal).

Actually, all of that stuff I just wrote, was me imagining the two laptop setup.
If you wanted to really go for using a single laptop setup then you could probably get away with running traktor and ableton together. You could attempt to use internal routing with the free app Soundflower, routing the output from traktor into one of live's channels. You might be able to sync the two using os x's Iac or Midi yoke if you're on a pc. Not positive about that though, but I think that traktor does have a midi sync feature.
There's a pleathora of other potential setups. Those are just a few that come to mind.
I'd like to hear of other people's setups as well.

Well, I received the Acer Aspire One today and can happily say that it runs serato just fine from what I have seen. I lowered the default latency by one notch (from 10 to 9) and that was enough to where it feels almost exactly the same as running Serato on my macbook pro.
Actually midi was the only thing I felt I had to lower it for because the control Vinyls felt fine even at the default (middle) buffer setting. Well maybe not needle dropping. But scratching, back cueing, etc. felt just fine.

1st thing I did when I took the Laptop out of the box was uninstall all of the crap bloatware that came pre-installed on the system. Then I simply did the common "optimizing xp for audio" steps and everything's good.
Oh also you have to go into your display, Monitor settings and uncheck the checkbox that says "hide un-compatible resolutions" so you can set your main resolution to one that is compatible with Serato. Simple as cake.

It's funny. I bought this thing to run serato on so I could run ABleton on my macbook pro at the same time but I'm almost considering getting a second one of these to run Ableton on instead. Their so small and portable and just powerful enough apparently. I'm sure you could only get away with a few channels of audio and not too many fx racks in Live on this little system but that'd still be fine for my usage.
That will have to be my next test :)