Don't bother with upping your brake size, just spend the money on some decent pads, I'd go with Pagid blues if I could afford them - think they are about £130.
bigger brake disks, pads and calipers mean lots more unsprung weight:
http://en.wikipedia.org/wiki/Unsprung_weight
which could ruin the handling of a lightweight car like the saxo.
I'd be willing to bet that 95% of the people who have upped their brake size have never tried any 'proper' brake pads first. By all means put on some braided lines and dot 5.1 but Its worth noting that alot of small production based racing cars use single pot pistons with pads like pagid blues and performance friction 97's which will both perform well from cold and mindblowing when hot, as for those who advocate 4 pots for an 850Kg car! they are alright for looks i suppose.
Also worth noting that standard disks and calipers with high spec pads will far and away outbreak a car fitted with GTi 180 disks and calipers with standard pads, plus if you increase the caliper size you also have to increase the master cylinder size so your costings must have to include new calipers, a new master cylinder, new performance disks, new performance pads, lines and fluid = big bucks for a negligable gain in my opinion