What you need is classic Marshall amp distortion in a pedal (as used by Slash pretty much forever). That will get you in the Slash, Black Sabbath & Offspring tonal territory.
Check these pedals out:
Marshall Guv'nor GV2 (a good pedal that will do the job)-
http://orangemusic.co.za/index.php?main_page=product_info&products_id=2471 or
http://www.soundz.co.za/ProductInfo.aspx?productid=MSGV2
BTW don't know which of the two prices is right, but I'd reckon R979 is the right one (based on what I've seen it in stores at).
Mi Audio Crunch Box (an excellent boutique-quality pedal) -
http://www.mieffects.com/cbinbrief.htm
You can get it locally from Music Connection in JHB. It sells for around R2000 I think, but a friend got his around 6 months back (on my advice and he's loving it!) for around R1500. It is well worth it if you have the budget.
http://musicconnection.co.za/news/?p=90
VOX Satchurator (a versatile pedal) - not quite Marshall-in-a-box, but quite close, plus it has a extra gain boost (more distortion) footswitch which makes it a bit more versatile than the other two IMO. These are around R1300.
http://www.voxamps.com/pedals/satchurator/
My 1st choice would be the Crunch Box, followed by the Satchurator, then the Guv'nor. All these pedals will do what you want and more and even if you can't afford the Crunch Box, the other two are still very good options. As a first pedal the Satchurator will probably cover the most different styles (even if it is a Joe Satriani signature pedal).
BTW a Tube driven distortion pedal uses a vacuum tube (as used in more expensive guitar amps) to help "generate" the distortion signal via various methods. Basically the idea is to create more tube amp-like distortion (or in some cases make you think they do). Some tube pedals are very good and some aren't, but they do tend to be pricier (normally R2000+) and require the use of a huge external "wall wart" power supply (except the VOX Cooltron pedals which can run on batteries). That help?
What amp you using?