Jamex are still around, but if you thought they were good you'll be well impressed by some decent suspension. For the money I absolutely recommend Spax RSX. If you're feeling like you want to splash out though KW Varient 2's are absolutely excellent, as are XYZ. All are coilovers fully adjustable in both ride height and damping.
Not sure about the Mk5 Astra but generally speaking Vauxhall seem to make strong enough chassis, so I reckon you'll be okay with just a front strut brace.
You want lightweight wheels too, the lighter the better. Heavy wheels will slow you down. Most importantly, a decent set of tyres. Goodyear Eagle F1's are excellent all-round tyres with a modest price.
When you've done whatever you have to do, take it to a garage and have the geometry set up properly (eg tracking) so all four tyres are flat on the ground. Normally when lowering the car tens to gain negative camber slightly, resulting in poorer traction and the insides of the tyres wearing more quickly.