Are you actually trying to download a flash video to save to your computer or just watch it on Youtube?
I assume "1.5mb Ram" is actually 1.5Gb RAM?
You don't mention what operating system you're using. If it's XP 1.5Gb RAM is ok.
If it's Vista or Win7 then it's borderline.
More likely your download speed over the internet is slow at your end or the server that you're trying to download from is slow.