Bryant,I also strongly concur with the others here! Although it might be feasible, trying to "go all across Europe" in a one month period is very unrealistic. With that type of travel, you'd mostly see Europe through a train window, with little time to appreciate the history or the people.
Thirty days is NOT a long time for travel, especially considering you'll need to allow travel time from city-to-city. I'd suggest that you choose a smaller number of countries and cities - those that interest you the most and relatively close geographically. Larger cities such as London, Paris and Rome deserve
at least 4-5 days each! I spent a full week in London last September, and barely "scratched the surface". There is SO MUCH to see and do! Keep in mind also that you'll lose the first day in flight, and will be "jet lagged" for a few days so you won't be up to optimal touring speed. For such a short trip, using "open jaw" flights would be a really good idea!!!
Some comments on your other points.....
> Global Eurail Pass - as the others have mentioned, this might not be the best option. If you have an idea on which cities you'll be visiting, you could use Railsaver.com to determine which combination of rail pass and P-P tickets would be most appropriate.
> Europe on US$70 per day - this is going to be VERY challenging, given the current exchange rates of the Euro and UK Pound compared to the US dollar! That's only about £35, which would probably cover your lodgings only. You should plan for at least US$100.00 per day and undoubtedly a bit more for the UK (this doesn't include rail pass or air fares). Since your parents suggested the idea of Backpacking across Europe, perhaps they'd be willing to provide a "Loan"
> Travelling with a Laptop - I've chosen to avoid this so far, although I've been re-considering lately due to the large number of Wi-Fi hotspots around Europe. A Laptop has a high "worry factor" especially staying in Hostels. There's also the weight to consider (do you really want to haul it all around Europe?). What do you want to use it for? If you were there for study or whatever, the situation would be different. There are lots of Internet Cafes, and many Hostels also provide these (although in some cases the Hostel computers are coin-operated). There are usually other facilities in the neighborhood, so you shouldn't have any trouble "to keep updated with family and friends". I normally use my Cell phone and keep in touch with family back home using text messaging. It's easy and fairly inexpensive (however not all Cell Phones will work in Europe, so that has to be considered also - if you're on a contract plan with either AT&T or T-Mobile, that might be one option?).
> Size of Backpack - that's somewhat a matter of personal preference. I've been travelling with about a 65L.
Eagle Creek Backpack (2003-vintage Continental Journey) for the last few years, and although it seems a bit "small" at times, it encourages me to
pack light! If you haven't bought a Backpack yet, it's
extremely important to be properly fitted, according to your torso length (any competent shop should be able to do this). Most of the weight should rest on your hips,
not your shoulders! I prefer a Backpack with a "stowable" harness system (to prevent straps from being torn-off by airport conveyor systems) and a detachable Daypack (which I use for carry-on when the main pack is checked). Whichever pack you choose, it's important to try it out fully loaded for a few hours to see if the fit works for you. I prefer not to have a pack that's too large (ie: heavy!) as I'm well aware that I'll have to carry it!
Although time is short, I would strongly recommend that you (and perhaps your parents?) read the Guidebook
Europe Through The Back Door. It's an essential "pre-read", especially for first-time travellers and will provide you with an enormous amount of information for planning and travelling in Europe. You should be able to obtain this at your local Library or book stores.
The month of May is not far away, so you'll need to plan very quickly!
Good Luck!