Now by the budget I'm guessing your a kid. sorry if your not. But well done for coming to PC. Now in the nicest possible way I could say this. What you need to do is to either buy used parts that are ancient, wish for magic genie, or save for longer. Also the new nvidia series is possibly being anounced soon and we have no idea what this will do for the pricing for the market.
Also tips:
Stay away from dell optiplexs
upgrade the Pc gradually
Don't right of newer DDR2
Don't expect to much
Learn the lingo and what to look for
Wait
And good luck as that task impossible.