I would like to buy a Thinkpad T420 with 8GB RAM configuration and install specifically 32-bit Ubuntu Desktop with PAE as I heard that 64-bit version has some issues such as providing support for applications based on Flash. Theoretically more RAM gives better speed, but practically there could be some issues such as consuming more power etc. Therefore, I am not sure whether I can get improved speed if I use Ubuntu 32-bit PAE with 8GB RAM instead of 32-bit PAE with 4GB RAM or not, specifically for watching streamed movies, playing videos, video encoding etc. Therefore, I am in dilemma whether to buy T420 with upgarded 8GB RAM or not?
PAE with 8GB RAM may be faster than a machine with 4GB RAM, by avoiding paging. PAE with 8GB RAM will also probably be slower than a 64-bit kernel with 8GB RAM, due to PAE overhead. Just how much faster or slower depends on what you use the machine for.
Flash works reliably on 64-bit Ubuntu, so it's no longer a reason to install a 32-bit OS. 32-bit binaries will just run on 64-bit Ubuntu without you needing to do anything special. So the short story is, today, if you have 4GB or more of memory, you should just install a 64-bit kernel.
Phoronix did some benchmarks showing a substantial disadvantage to PAE on some tasks.
More RAM uses more power, and gives more opportunities for something to fail. The main benefit is that with more RAM, your machine needs to do less physical disk IO, but this is only a benefit up to the point that all the data it touches fits in memory. So, there is no point installing more memory than will actually be useful to fit your typical working set.
The first two don't need a lot of memory; only enough to make sure your browser and video player are not paging. The second is mostly cpu-bound, not ram-bound.
So, either 4GB or 8GB memory would be fine. If you expect to do development or other more memory-intensive tasks later, get 8GB. Either way, install a 64-bit kernel.
The AMD deck "Myths and facts about 64-bit Linux" from 2008 makes these points:
Since 2008 most applications will be primarily developed in and tuned for 64-bit mode.
PAE is useful when you have 4GB RAM or more. In case if you have no problem with specific software you may have, try a 64-bit OS.
PAE does not improve speed but gives you the chance to use more than 4GB of memory. The performance impact is very little actually and you will only "feel it" if you start using the memory above the 4GB limit. Basically if you have a 32Bit PAE Enabled system with 8GB RAM the performance impact will only come AFTER an application starts using the memory above de 4GB (This is the one from 4GB to 8GB). But the impact is very little. You will not notice it. What you will notice is 4GB of more ram.
Also for a system with 4GB of ram is not needed or recommended to use PAE since you are not going over the limit which is the 4GB of ram (which is a 2 by the power of 32 in bytes)
For videos like flash games, flash youtube videos, etc.. you won't notice anything different from a 32Bit with PAE or a 32bit without PAE. It will be the same performance.
Lastly, the performance and issues associated with the 64Bit Version of Ubuntu last i heard were solved with several flash updates (Like the Flash 10.3 version) so if you install it right now and then install the flash version of adobe you should not have any problems.
Now for the "Should I buy or not buy the laptop with 8GB or ram". Well if you are like me on my home that the things i use it are to listen to music, see movies in HD quality, download stuff, open like 10 Libreoffice documents to work on, open Firefox and Chrome to work, etc.. then you do not need 8GB of ram. With 4GB of Ram you should be not just fine but happy (since you did not overspend on 4 more GB of ram to not use them).
Now if you are like me on my work place and you like to compile stuff, configure servers, render images and videos, etc.. Those are some heavy usage cases. So you need more ram for them to operate faster. Rendering something with 4GB is not the same as 8GB. Same for compiling (with the appropiate flags) and for the rest of the intensive cpu/memory apps.
But for the normal work, video watching, typical downloader and common user 4GB is enough.
Hope this helps.
Install 64-bit version better, it gives some performance improvements. With Ubuntu 11.10 even better because it is based on the new Debian multi-arch which means that it can install 32 and 64 bit programs on any architecture so there are no disadvantages in the 64-bit system compared to the 32-bit.