[XeTeX] Four Initial - General Problems

[snvv]

Hello
Recently I started using lattex and I have four problems.

1st. When I use the xy-pic package to produce some graphs the quality of the 
graph (especially the circle) is really bad. On the other had using tex the 
output is really nice either as dvi, ps or pdf.

2nd. When I type a formula (i.e $\left\{ y_{t}\right\}_{t=1}^{\infty}$ ) the 
letter is not of the center of {}.

3nd. How can I use a different math character set? I have find the 
\setmathrm{Minion Pro} but then i don't have bold, italics etc. I would like 
something more general.

4th. How can I use the tex fonts? Now I can use all the ttf fonts but none 
afm, tfm, type1 etc 

I apologize if the questions are really simple, but I don't have solution even 
after extensive googling and reading.

Thank you in advance for your help.

Sotiris

PS. I use Linux debian sid with texlive.