Yes, removing the rear panel will allow access to the memory slots which are centre-top of the laptop. On the Vista front, i suggest you give it a go and update to SP1 as soon as you can. That has helped with performance but also, you can turn off a lot of unnecessary services Vista boots as standard, optimise startup/registry using certain software and possibly disable the transparency effect and other pointless animations (e.g. combo-box) to save more of the CPU.
I say this because loading XP will be more of a pain because you'll need SATA drivers and the only way XP setup allows for third-party drivers is from a floppy drive...which you obviously don't have so you'll have to slipstream the drivers into a new copy of XP to get around this.