use it as my main domain for SEO purposes and redirect to my old domain
Which are you considering doing since you don't seem to know what your plans are?
1. You have an exisiting domain.
2. You have the possibility of buying the new domain name.
a) You could buy the new domain name and point it at your existing domain. The only benefit you would gain from this are type-ins, nothing else.
b) You could buy the new domain name creating a single splash page with all links from it to the existing domain. You must ensure that the splash page has unique content and not just a copy of your existing index/home page.
c) You could buy the new domain and upload the exisiting site "as-is" and then point the old name at the new name. Do this at your name registrar level for ease and maximum effectiveness and if you have a cpanel you can do it yourself.
This would retain all your existing inbound links and all the search engines would soon pick-up on it and within a few weeks the old site would be purged and the new site ranking as before and if it is a widget keyword domain may actually rank slightly better...no promises on that one:-)
d) You could buy the new domain and create a completely new site, do not create a duplicate it will go nowhere except for a type-ins.
If the new name is more applicable to your "service" I would be tempted to go with c.