Can someone help and guide me how to travel from Brussels to Italy and back from 1/05 to 04/05?

I want to plan a trip from Brussels (Belgium) and see rome and venice. the trip dates are 1st may 2008 to 4th may 2008. journey via train, bus and flights will do. should not be more than 450 euros

Assuming you mean 450 Euros for the travel costs, you can fly to Rome on May 1st via Sabena, take the train to Venice on May 3rd, and fly from Venice back to Brussels via Sabena on May 4th for a total of about 386 Euros. You will need the rest of the 450 Euros for travel between the city centers and the airports.

Airfare: About 285 Euros
Train: About 101 Euros

I checked RyanAir, but their fares R/T Brussels-Rome were the same as Sabena (they don't fly to Venice). EasyJet's website won't let you select flights from Brussels to Rome, but it appears their fares are only a bit better than Sabena (though you might check with them by phone).

Taking the train the whole way would be more than 450 Euros for the round trip, even if you could accomplish the whole trip in less than 4 days.

You won't make it by train or bus in only 4 days; you'll have to cross almost the whole of Germany to get from Brussels to Italy. If you had more time, I would have recommended the Eurail Select Pass:
http://www.eurail.com/eurail_select_pass_3_countries?currency=eur
That would cost 319 euros (1st class adult).

I found a flight from Brussels to Rome and back at http://www.ryanair.com for 214.90 euros, fees and taxes included, but you would have to return to Brussels on May 3rd, as there are no flights available for that day. Maybe you can use another airline; but Ryanair is considered to be among Europe's cheapest.

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edit:
I didn't check out a return flight from Venice, but thankfully Barry did.