What's the best way to play Pokemon Diamond/Pearl?

Hmm... well if you do decide to purchase a DS, you might want to wait a while. I think the DSi is supposed to be released soon, and that'll probably make the DS Lite's cheaper. You could probably find one for $100 someplace.

There's always eBay for deals. I would recommend against an emulator and rom, however, since aside from being illegal you're keeping profits away from the game manufacturer and discouraging them from making more games. Granted Nintendo will never stop making Pokemon games but it's the principle.

Well I'll give you my good old opinion Dialga/Palkia-You can catch it simply with a Poke'Ball. Uxie/Azelf-I would go with an Ultra Ball. Mesprit-Stop him with Mean Look and use an onslaught of Quick Balls. Regigigas-Timer Balls work the best on freak. Giranti-I would try to catch him in a quick fast Ultra Ball onslaught. Heatron-I caught the loser in a Great Ball Sorry if I missed any. Cressilia-Master Ball time baby, I couldn't catch the prick without it.

you really do need help if your still playing pokemon games. just kidding. You can get it cheap on ebay, they sell the Wii there for 100 bucks, about 30 bucks for a used one. I don't know about the d.s. though

Alright, kid.
I don't know many adults who like to play Pokemon. I don't know any 15 year olds who like to play Pokemon, but to find a DS, go to Best Buy and they sell them for $85.00 U.S., and they might charge you to get the internet configured.

Have a Nintendo DS and Wi-Fi.