I bought a piece of 3x5x1/2-inch plate from the scrap yard for scrap price... It weighs about 305lbs. (I about $47 for it)
The same piece in 7/8-inch plate comes to 459 pounds.
Now I'm no building contractor, nor do I play one on TV, but I'll still go out on a limb here and say that you're floor isn't set up to hold that kind of weight. Since a dining room table is usually bigger than 3x5, you'll be looking at nearly 1,000lbs with the 4x4 "minimal" frame, the I-beam and the top. Throw another 1,000 pouuds for dinner guests (I go 220 myself, my 13 year old is about 110, wife is 135, so that's that's nearly 1/4 ton for a family of three), and you'll be sitting in the basement before the main course is served.
Do a google search for Steel Weight Calculator, and you'll find all sorts of sites that will give you the weights of plate, I-beam, tubing, etc.
I think your big, burly dining room table would do better as a welding/fab table, so you can whip up something lighter for the house.
-Brad