I think the Renault Sport ECU everyones been on about recently is the one for the Cup - which means it wont work with a Mk1 172 as it will be for cars with a fly-by-wire throttle, as opposed to our good-old cable throttle... Besides, knowing how quickly these Cup upgrades have started to appear (not) itll probably be a good 6 months before anyone gets their hands on one anyway.
As Simon mentioned above, Renault Sport already do a Group N ECU for the Mk1 172, and yes Simon172 does have one and was trying to sell it a while ago - so it might be worth talking to him if youre interested! He reckoned that he could hold off an Escort Cosworth with that ECU fitted due to the extra revs and power available - and I dont doubt its true either! Sounds good, but I think it was about £320 or about that... He also said that it felt a bit strangled at high revs asif it needed a different exhaust, which is worth bearing in mind if you decided to go for one.
Matt