So then which is better? A4-4000 and A55 mobo or 5350 and AM1 mobo ? For basic tasks like web browsing and video watching ofcourse.
5350 for several reasons:
(1) Other than gaming, the performance is almost entirely in favor of the 5350 especially in multi-threaded scenarios. There are extensive benchmarks (synthetic, browsing, gaming, etc) comparing both the A4-4000 & 5350 (among others) at this review:
http://www.techspot.com/review/806-amd-kabini-vs-intel-bay-trail-d/page3.html
(2) For uses such as yours, power efficiency is an even greater consideration since both chips are more than adequate. The 5350 is vastly more power efficient while under load and draws less power at idle. See page 8 of the review mentioned above. The 5350 idled at 28 vs. 31 (4000), CPU load 44 vs 72, and CPU + GPU load 51 vs 86. Those for the 5350 may be high as well. I'm idling a 5350 with a 240gb SSD, 2 5900rpm Hard drives, and a PCI-E ethernet card at 28w (Windows Home Server 2011).
(3) Since power efficiency is a greater goal given the circumstances (usage is more than satisfied by both chips), the AM1 board provides for a (speculatively) greater upgrade path. The FM boards will not be the socket for the most efficient chips. IMO, the AM1 will be. Thus, Beema will likely end up as a AM1 socketed chip (no guarantees of course as AMD has not announced such).
(4) GCN - the 5350 has the GCN architecture so it has some future upside as more software takes advantage of it.
(5) The 5350 will support Mantle which may provide additional gaming gains in the future for those games that use them. Again, though, neither chip is going to be adequate for intensive high res gaming.
(6) Last, but certainly not least, an A55 mobo is limited to SATA 2. The 5350 supports SATA 3. If you use an SSD, or may in the future, you want SATA 3.
Even if you consider the A4-4000, you should not buy an A55 for the lack of SATA 3 support. Also, many A55 mobo's will not support an upgrade path to Kaveri. Your safest bet is an A78 or A88 mobo if you go the A4-4000 route. See chipset/cpu support chart at
http://www.anandtech.com/show/7677/amd-kaveri-review-a8-7600-a10-7850k/8
Hope this helps.