BlinkY,
Look at the HP16C or similar.
It does Hex, Octo, Binary... It can do math in integer or floating point - integer like a real microprocessor without a floating point co-processor would do. It can do logic ops like AND, OR, XOR...
I think you will find that kind of calculator very useful.
I was going to suggest that, but you beat me to it. Incredibly long battery life, great display, simple RPN. You can program it to convert between hex and floating point, using virtually any floating point scheme. It can be set to do integer arithmetic with any arbitrary word length, using ones complement, two's complement, or sign/magnitude, and it can display results in decimal, hex, octal, or binary. Programmable with all the standard boolean algebra operations. Only down side is that it doesn't have trig functions. I bought mine in 1984 and it's still going strong.
There are several software emulators of the HP16C. They lack the long battery life, great display, and solid keyboard, but are otherwise good.
But I'll also agree with those who suggest that a calculator isn't super-critical for CS work. Even if you get the HP16C, you'll probably want a fairly standard engineering calculator to get through the math and other prerequisites. And the HP16C functions can be done with the right software on a full-sized computer. So while I think the HP16C is a great calculator for anyone doing programming at a low level, it's not absolutely essential.