Come on - the car tax is just an excuse for two new cars!
It's going to take you a long time to recover the £400/year car tax fom the money you spend on these two babies.
The two options I love are:
Bi-Xenons - a 'must have' in my opinion as they make such a difference. They are impossible to retro-fit if you change you mind later.
Bluetooth/USB - the 'hands-free' phone is obviously good, but the convenience of having 1000s of music tracks on one small cheapo USB stick is brilliant. You also get the MFSW and cruise control too!