WILL I FEEL ANY DIFFERENCE WITH 18'S
 

As above im upgrading my wheels from 17's to 18's

does it make the acceleration slower? (people saying alsorts)

Unless they are very heavy, the acceleration depends on the tyres you fit !

Obviously you need the same rolling radius if possible....

215/45/17 = 215/40/18 with a bit of error, law allows 10%, mine are within 1% of my sat nav reading.

If you choose a good quality aftermarket 18" then they are likely to be lighter than your standard 17". You need to make sure you have good quality tyres, its all about the tyres in my opinion. I tried some Sti 17" 225 wheels ( very heavy ) with half worn Bridgestone Potatoes and it tramlined like mad. I now have Rota 18" rims with 225/40 Goodyear Eagle F1's and the car feels and looks a lot better than with the standard 17"'s or the Sti 17"'s. Going to 19" rims may well give you handling issues but 18" on a newage will be fine.

Cheers thats what l wanted to hear although the tyres will be 225/40/18

Then you have to consider the width if it's been/being lowered...

I went from standard wheels on my 04 wrx to 18 iinch oz superleggeras.They look awesome but the car tramlines alot.So I'm hoping when i fit eibach springs and prodrive geometry set up it will be better,if not off there coming!

Geometry should help. I had mine done at Powerstation and its a definite improvement. Tyres are the significant factor for tramlining. I've never felt it with the F1 GSD3's on. Also what width tyres do you have? Standard 17" wheels are 215/45/17 and the move to an 18" should be to a 225/40/18 tyre.
18" wheels with an 8" width as opposed to 7.5" width using 235 tyres would be more likely to tramline. Worn suspension could be a factor though seems unlikely on a 2004 wrx.

I have 19" wheels on my 03 WRX altho it has Tein springs and ProDrive geometry set up I still think the handling is compromised (I have only had it 2 months and came from a very well set up FTO) so I am thinking of p/x in my wheels with some 17"s or maybe 18"s to get a bit of the handling back but still fill the arches a wee bit!

I also had F1s on the FTO for years until the last change I went for Dunlop Sports Maxx and they were really good - I think I'll go back to the F1s for this Scooby tho as I know the tyre.

my 18s were lighter than the standard 16s.
road holding is way better

The tendency is for bigger wheels to tramline more and for the ride to be a bit harsher as there is less rubber/air between you and road. As other have said, the acceleration will change if the rolling circumference is different to standard.