Here's my gear:
Jackson JS35K guitar(crappy one but beautiful as well)
Line 6 Spider II 30Watt amplifier

I like very much hard stuff like Amon Amarth Dark Tranquillity Slipknot...
So,my point is that i want much more distortion caz I don't like my amp's.
I've seen a very good Boss Distortion-Overdrive Peda.The Boss OS-2.
U can see it rigt here
Is it good??I don't want to spend too much money,but it seems like a very good choice.
Moreover,i'd like i wah pedal too,but since i play the guitar for a year only and i'm not such good at writng my own solos i'm thinking of buying Line 6 FBV Express.It changes the amp's chanells and it has wah as well.Are they a good choice???
I would go for A ds-2 instead of an overdrive pedal (based upon what you said you wanted for distortion.) As for the wah...I'm not a fan of line 6 wah pedals but if it is also used to switch the channels on your amp I would go for it.

Wait....the distortion of your line 6 isn't enough? I'm pretty sure that your gonna need a new amp if your unhappy with the sound. I pedal really isn't gonna fix it for you.

Adding more distortion isn't going to increase sound quality!
The spider has a lot of distortion.....it's just that it's thin, buzzy, and has no balls. The amp is just ****.

A distortion pedal will not fix your problem. You need a better amp.

What's your budget?

If you're really set on only a distortion pedal, the boss MD-2 is a good bet.