1 It's a Dell
2 What type of motherboard can I use?
3 What type of graphics card can I use?
4 Is it ok to put 2gig of ram?
1 Because you have a Dell there is a good posibilty that the motherboard is a non stanard fitting within the case, so first of all you might need a new case.
2 The best you can afford, depends which type of cpu your want to buy, in you price range I would think an AMD Athlon.
3 Depends on the new mobo specification, it will probably be 8x AGP. Once again you will probably be governed more by price and whether you want ATI or Nvidia chipset.
4 2Gb of ram seems a bit excessive to me compared to what else you want to spend, games players i'm sure will advise you here. I would have thought slightly less on the ram and more on the graphics, but I'm not a game player.