Since I don't normally have a spotter unless I catch some of my friends there I've been using the Smith Machine thing for benchpress.

How much weight does it take off? Like if I did 250 x4 today, what would that translate to normally? And is the Smith Machine effective at all for gaining strength, having no spotter I don't want to try and max out without being safe, but if it's just a waste of time I'll stop.

It's not a waste of time- You can still get a hell of a chest workout on a smith. The only reason free weights are better is because other muscles are called into play to stabilize the weight- Yes, it's better to work with free weights so those stabilizers can get worked- And yes, with stronger stabilizers, you'll be able to bench more. Free weights are better- But the smith is definitely not worthless at all. You're using it for one of the best reasons- Safety. It's better to get a workout on a smith machine than to not work out at all, or even worse- Try lifting without a spotter and get hurt.
There's no way to know how much weight the smith "takes off", because the machine itself doesn't really take the weight off- How much you'd be able to bench with free weights depends on the strength of your stabilizers as well as chest strength. Perhaps your stabilizers are stronger than most- Perhaps not. I definitely wouldn't recommend getting on a free weight bench and trying to lift the same amount of weight you do on the smith, though.

With the Smith machine, you would normally have a 20K bar, and i think each hook is around 5-7k.

These can be added to your total, as you are pressing them. One thing i would be wary of is that the Smith machine moves along a set path, and not the natural path that a barbell takes. Be careful when adding weight incase you strain something down to this, and watch your development incase some areas of the chest are gaining disproportionately.