Your notebook would be standard 54 Mbps (theoretical max) 802.11 g. So unless you get an add-on card which matches the 108 Mbps method utilised by a "go faster" wireless router, you will not get the much greater throughput.
So I think you ought to stick with plain old 54 Mbps 802.11g kit throughout, unless your environment demands greater signal coverage.
Also, my own experience of setting up a wireless router connected to a modem/router was much more convoluted then getting a single box modem/wireless router setup.
There is hardly any price differential between them anyway so if you're not confident, I'd go for a single box to replace your existing modem-non-wireless router.
Hope that helps a little.