I grabbed two Attack on Black Reach boxed sets, then added a Battlewagon (christmas present), Warbiker boxed set and a nobz boxed set. I carefully mixed and matched nob bits so that I had 5 with big choppas and 5 with powerklaws, plus 5 with normal weapons. Add in a painboy and you can build a mob of nobz for any points value. I did all that back at Christmas, and have been using them, and not buying any more, in order to concentrate on painting those units before moving on.
Some general tips would be:
1) Get used to losing a LOT of guys. I got told, "You need to varnish them, because you pick them up in handfuls", heh. This is why you should not run boyz in mobs lower than 20.
2) Orks paint up well, and people are not used to seeing fully-painted mobs hit the table.
3) You can do a lot of shooting if you want, using Lootas,
SAG, shootas, warbikers. This works on the theory that enough bullets will eventually overcome your terrible
BS.
4) Don't let anyone charge your units. Orks don't take a charge well. It causes mass death and you'll lose the mob on sweeping advances more often than you'd believe.
5) Powerklaws seem to be very key to Ork melee strategy at the moment. Sad, but true; entire mobs of boys are little more than "Nob delivery systems". Get involved with the strength 8 carnage.
6) Watch out for Hydras,
LR Crusaders, and the like; stun them with rokkits, and ram 'em with deffrollas.
7) Don't send your mega-armoured nobz against terminators; it won't end well.
My army, that I have used with reasonable success for the last few months, consists of:
Warboss (powerklaw, cybork, boss pole)
Nobz squad (including 2 Big Choppas, 1 Powerklaw, Painboy, all with cybork - a very budget squad, with relatively few upgrades, but they do the trick in 1k)
Both are transported in the Battlewagon, which sports a Deffrolla, Kannon, Big Shoota and Red Paint Job.
Two mobs of 20-strong Boyz Mobs, each with Powerklaw and Bosspole equipped nob. They are currently both choppa boys, but I've been testing one as a shoota boyz mob, and that works well.
Three warbikers, with Powerklaw and Bosspole equipped nob.
Two Deffkoptas, one with a buzzsaw, both with
TL rokkits.
This slots in at exactly 1,000 points. The deffkoptas tend to die horribly, but can occasionally totally screw your opponent. The Warbikers are really too small a unit, but they can really put the hurt on enemy squads with a good bit of shooting. The boyz mobs do the job, which is about all there is to say about them, and the nobz and warboss can reliably take on most anything. Yesterday I played a game against an all-biker Space Marine army, and they chewed through the entire enemy army in four turns of melee.
You could try taking a look at
http://www.the-waaagh.com/, which is a dedicated Ork forum. I found them to be a big help when constructing the above list.