The dual-core atoms are reasonably powerful; but AMD's (finally!) come
out with their 'Atom-killer' as I've nicknamed it.

(I'm running an Atom D510 w/NM10 chipset in a MSI Wind barebones box;
it is currently serving as my IRC shell, local ubuntu mirror, and
Minecraft server with 4GB of RAM total; works well in that capacity as
long as I'm fine with 2 HDs as my max unless I hang more off USB2.0
ports)

Here's MSI's examples:
http://www.msi.com/product/mb/#/?sk=BGA%20FT1

The AMD 'Hudson' (Llanos) APU (Accelerated Processing Unit; combined
CPU + GPU on one die) - the E-350 model is a 1.6GHz dual-core part;
with an HD6310.

The CPU itself has been benchmarked at 40% faster single-thread
performance over an identically clocked Atom!  Doesn't have the HT
that some Atoms do - but isn't a crippled part.  VM extensions, 64-bit
capability, support for 8GB of RAM and Gigabit Ethernet all
on-chip...and since it has more performance capability, it runs less
on long-thread stuff than the Atoms - but then goes back to a
crazy-low idle; coming in better in overall powerdraw from a few
reviews I'd read.  (Intel's Atom options generally force you to pick 1
of the following: More than 2 SATA controllers, Gigabit Ethernet, or a
1x PCI-E slot; then also on the CPU itself a choice between VM support
and 64-bit support; either or but not both)

The GPU inside (HD6310) is supposed to be about 50% more powerful than
the IGP that has been built inside the 780G/880G chipset
(HD3200/HD4250) - which I can say myself games reasonably well (runs
all of Valve's Source-engined games just fine on low settings;
Left4Dead 1 & 2 are the only ones that could use some improvement)  If
I had the spare budget for it, I'd be trading up now!

So I'd consider buying the $250 barebone case; then buy one of the AMD
MiniITX boards; and get more power out of the system.  (IF you're
looking for even more power, you can get normal desktop chips that'll
fit in MiniITX boards for normal uses and put them in there; both
Intel and AMD-based)