Matt has already mentioned that battery useage is a lot better than it used to be. Also, buy a set of 4 AA rechargeables, have all 4 charged and if 2 go flat, you've got a second set and you can keep shooting.
Edit: didn't realise but Matt's also already suiggested that!
I started out with a very similar camera to the SX120 as well (Powershot S3 IS) and was always very happy with it.
Again, Matt has already said that memory cards aren't expensive. Check out these
2GB cards for £5.99.
Cases also aren't expensive - WEx will do you a bundle of the camera and Canon's own case for £175 (
link here). The Canon own case is only £15 and you will be able to get ones cheaper than that.
As for a tripod, it's not an essential item. You might want one later on but there is nothing wrong with resting your camera on a wall or the floor. You can even use a bean bag which costs pennies and they stabilise the camera really well.
I must admit I was surprised to see that the SX120 IS had fallen down to £160, it's a bit of a bargain as the RRP is still £250!