A desktop doesn't have to make concessions with components that laptops have to so I think those two you picked, while I don't really know the details and the component market anymore, seem more than powerful enough to do what you want. Start adding components and if you need to compromise then you can reevaluate your choices later.
This site makes finding and combining parts for your system easy:
http://pcpartpicker.com/
Found this article, quite recent that seems right up your alley. The $600 system they put together seems quite good:
http://lifehacker.com/5840963/the-best-p...0-and-1200
And another few $600 build for a gaming pc rig:
http://www.tomshardware.com/forum/364155...ld-dollars
http://www.reddit.com/r/buildapc/comment...ild_for_a/
http://toptengamer.hubpages.com/hub/Best...uter-Build
Gaming PC focused builds (even on budget) will tend to be well suited for an artists workstation, as they emphasise performance and graphics which is what you want. When getting my next laptop many of the top contenders are gaming focused machines, or the "mobile workstation" class.
You might want to do a quick search for off the shelf $600 dollar boxes just to compare specs. For the most part however, you'll find they probably use integrated or shitty video cards because they are built just for light use and your average user probably wouldn't notice.
General notes:
Definitely put the money into the CPU, RAM and Graphics Card. I think the graphics card is more important if you are doing 3D or gaming. I don't think 2D work is actually as resource intensive. But yeah it is still worth getting the best you can.
For the RAM, I would go with 4GB at a minimum. 8 GB if you can stretch it. I think anything below that would be already scraping too low for what you want, especially if you intend to paint while multitasking (streaming music, livestreaming etc etc)
The hard drive can also become a bottleneck, so a 7200rpm speed spin drive is desirable, and if you can find a really good deal on a small SSD (solid state drive, maybe second hand) for your OS and apps it will really improve the overall speed of your boot times and file and application operations, but it is not necessary to run what you want just fine.
If you plan ahead you can always upgrade. Think about the future and if you think you will add components and upgrade as time goes on, make sure you factor this into the choice of the tower itself and the motherboard chipset just to see if something new is coming online and how that might affect future components. For example if you wanted to get on board with the new Haswell Intel chips in the future that boast a lot less power usage and additional graphics prcessing, you would need to have a totally different mobo. Of course Haswell is probably better for laptops and gamers so it's not much point in your case. I'd just say make sure your mobo has enough slots for a good upgrade of RAM. Upto 32GB would be nice if you want a boost to performance relatively cheaply. :)
To give you some idea of running speeds. I was using until recently a 7 year old Dell XPS m1530 "gaming" laptop. I maxxed out the RAM at the time: 2.4GHz, 4GB RAM, 128MB graphics card, and I was painting 4000+ px canvases just fine. It did however stutter when also trying to screencast and stream music at the same time, which ruled out livestreaming for me. Another factor was that I ditched Windows and wasn't using Photoshop. The system started running really slow almost to the point of ridiculousness a couple of years ago. I transitioned to Ubuntu and I could magically use the machine again faster than before and got 2 more years out of it with no decrease in performance until it was stolen. Probably could have pushed it another year. Goes to show what a resource dog Windows actually is over time.
Hope that helps
