Amd 4 core or intel 2 core

IMO they're about equally good CPUs. But don't forget about the AMD dual cores either.
Quads: X4 620 / X4 630 Twins: X2 250 / X2 550 / C2D E6300 / C2D E7500
For the tasks you list you would probably be happy with Athlon II X2 250 Regor 3.0GHz or Pentium E6300 Wolfdale 2.8GHz. Both would make very good general purpose computers.
For a bit more you could get Phenom II X2 550 BE 3.1GHz or Core 2 Duo E7400 Wolfdale 2.8GHz.
Athlon II X4 620 2.6GHz vs Phenom II X2 550 BE 3.1GHz? I'd go with Phenom II X2 550 BE 3.1GHz.

If you're budget conscious and looking at the $100 price area, I'd check out the x4 620 as advised above.

It really depends on your budget and the purpose of the build.

In this case, the Intel CPU will have the disadvantage of being on socket 775, a dead end; the AMD solution will be on AM3, for which future CPU upgrades, if needed, will be more readily available. If you're willing to go to $120, get the 720BE. If not, start minimally with a X2 240 or 250 knowing you can upgrade it later if needed.

personally I would go with either the phenom x2 550 or the athlon x4 , I can also vouch for the regor core athlon x2 series as being excellent chips

+1!!!
im a HUGE fan of those quad cores! cant wait till i get some cash to buy one of those cheap monsters and overclock the hell out of it.

yeah... i would take a low wattage sempron over a quad for basic internet.

i have both and its a toss up. have an core 2 E 7200 clocked to 3.0 ghz and an amd 710 triple core phenom II @2.6. with the same ati hd 4850 they get about the same fps and cost about the same, around a hundred bucks. 7200 are not available anymore but 7400 are. pick what you like, for 100 bucks the results are gonna be about the same. but..am3 is more future proof.