Help needed to decide for Canon Wide Angle and Telephoto lenses for EOS 450D

[ubk1004]

Hi,
I am a newbe in DSLR. I have recently bought a Canon EOS XSI with 18-55mm IS AF lense. I found the lense pretty good.
Now I wanted to have a wide angle lense and one telephoto zoom lense to learn the art of photography better.
As you can understand that the new xsi costs me a lot, so I am pretty tight in budget here.
I have searched through various sites about Canon, Tamron and Sigma lenses. But the more I searched the more I became confused.
Could somebody help me to get out of this trouble.
If possible please suggest me 2 lenses (one wide angle and the othe telephoto) for my XSI among canon, tamron and sigma in low budget which will help me to learn better.

Any help will be greatly appriciated.

Thanks

UBK

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