~ I have two notebooks : one with windows xp sp2 + linksys wrt54gc and the ~ other one has Vista premium + core 2 duo so it is equipped with wifi ~ (a/b/g/n). ~ ~ I have linksys router - wrt 300n which provides internet connection for my ~ laptops and allows them to be seen to each other. ~ ~ I have noticed that the speed I can copy the files between my notebooks is ~ as low as ~500 KB/s - 900 KB/s which I think is too slow.
What makes you think that that is too slow? Have you connected your two computers up via a 100Mbps or 1Gbps wired connection and seen different results.
~ I regard the ~ proper speed should be at least 3 MB/s.
I disagree. If by "3 MB/s" you mean 3 * 2^20 bytes per second, i.e. 25.166 million bits per second (which I call "Mbps") of application layer throughput, then that would definitely be an improper (excessive) transfer rate for your network setup.
Let's assume that you can achieve in every case 54Mbps 802.11g with absolutely no errors or interference. And let's assume that your computers' CPUs, disk drives, and SMB/TCP/IP stacks, etc. are not in any way a bottleneck. Then your very best case application layer throughput for a data transfer through this medium will be about 25Mbps.
However, bear in mind that each packet will need to traverse your wireless channel twice, once from the source PC to the AP (wireless router), and again from the AP to the target PC. So, this will halve your best-case end-to-end application layer throughput, to about 12Mbps.
~ The signal strenght is "excellent" ~ and the notebooks are in the same room. I use DHCP. ~ ~ Please, give me a piece of advice what to do to increase this dramatically ~ low speed.
You present no reason to believe that you should experience any better throughput that what you are reporting.