Loosely speaking it would be possible to get fair results in Photoshop, but you'd need to know what you're doing and Photoshop isn't the kind of software you can just open and use... it takes years to learn (I've been using it for years now and still feel like I'm only scratching the surface).
As for the top-down view with some perspective distortion, you'd need a 3D program and a good rendering engine to to get that level of realism. I think in this day and age it's quite easy to fall into the trap of thinking that software produces results; in actual fact, it's the people who control the software that produce the results.
I've recently been toying around with the idea of a Borderlands themed keyset and I decided to build a 3D model from scratch using Maya for the keycaps mesh and Mental ray for the rendering. I know what I'm doing with the software and it has still taken me several hours to get to the stage where I am now (see below). As xondat says, if you want to design a keyset you're best bet is to collaborate with somebody who knows how to use industry standard graphic design software (like Adobe Illustrator/Photoshop) and also 3D software too...not to mention the time to put everything together!
In my lowly opinion, a keyset doesn't look good until it looks realistic, and then you can make a judgement on whether or not it will work IRL.
