320x240 is only good for big simple video graphic and logos ...
768x576 is ok to start ...and uses less video graphics power ,but looks like crap next to 1920x1080...
So if your Mac works good running two HD video with video EFX,s ..then bigger the better

Cool thanks

320x240 is gonna look absolutely horrible and you should stay away
768x576 is a little better than standard definition ... which yes compared to a full 1080p video is not gonna look as clean however the way the files are encoded make all the difference

What model mac do you have which will give a good idea on which you can run properly

The "norm" as of lately is actually 720p and what is what most pools are releasing

Also do not waste your money on Serato Video look into Mix Emergency (www.inklen.com) it runs a lot better has better features and is the most stable solution out there

As far as where to get your content from look into www.smashvision.com as they are the leader in quality hands down ... Other reputable places to get your videos from would be Promo Only and Xtendamix (www.xtendamix.com) (as long as your in Canada or the UK)

I personally go for the highest quality file that I can get so if 1080p is available I will get that but if 720p is only available I will get that ... The same goes for older videos if a 640x480 or 640x360 video is only available I will get them ... Do your research and find out what your computer is able to handle and get the highest available ... But remember not all 1080p, 720p, Etc. encoded files will be the same you can easily get a 720p file from a non reputable pool that will look worse than a 360p video from a reputable pool

I'm on a 2011 Mbp with 10gb ram and 2.3ghz i7, 1gb nvidia. I got sdj video free so am using that for now to see how I get on.

I'm not really bothered about mixing music videos, but more playing visuals do have got beeples loops and some qtz files too.

Any advice about the second screen?

You should be good to mix those no problem but if you're only looking to add visuals I'd still look into Mix Emergency as that will run independently from SDJ so you're not loading up a different visual every 3 minutes (unless that's what you're wanting) as far as the output there are a lot of different options you have ... There are adapters to go directly from mini display/thunderbolt to VGA & HDMI however those aren't your only options there are a range of different scan converters to be able to get you to component, composite & even S-Video so depending on the inputs on your screen that will dictate what hardware you'll need but most commonly if you're not going into an HDMI screen you'll need the minidisplay/thunderbolt to VGA and a scan converter

Ok will look into me I think. What does the scan converter do and why is it needed if not using hdmi?

The scan converter is for connecting to a display that is either Component, Composite or S-Video the reason it's not needed for HDMI is because apple has a connector that goes directly from minidisplay/thunderbolt to HDMI same as VGA

Cool - thanks for the advice all who contributed. This helped me out too.