Well I assume you're reffering to T5's versus HO-T5's (High Output T-5). You can veg under normal T5's, but with the prices on florescents getting smaller each day, you might want to put in the extra couple bucks for a HO lamp. I currently veg/flower under my 4' - 6 bulb HO T5 from Hydrofarm and it's been treating me quite nicely. Here's a couple from my T5 grow:
As far as space, it's really dependent on what method of growing you're going to use. If you did a SOG setup, you would need much less space than if you were to just flat-out grow the girls and top them (making them far more bushy). You could do ten plants in as little as a 2' x 2'x area if you really wanted to.
Are you doing hydro or soil? That will also help us help you.