Buy if you need them now.
IT/PC Show is usually a good time to buy because of the freebies (I remember I bought my 300D just a week before one of the shows (at that time I 'needed it NOW'), and they were giving away 75-300(?) lenses as part of the kit
Buy if you need now is good advice, but if you know you need it 2 months later for holiday, it's good to have time to get used to the camera too... (and I don't think the price of the 350D will be much different in June. 2nd-hand 300Ds should be going for less though ).