Even if you were on 1 Megabit, 1.5 Megabit - or even 2 or 3, a PCI slot 10/100 LAN card would not be breaking into a sweat.
Any problems are more likely the DNS servers or the proxies, and for HTTP/FTP downloads, get a download accelerator that can do 2 or more streams (actually, more than 2 is often unproductive, especially for many FTP's where the extra threads will just fail, or may even get you an IP adress ban for "hammering").
Fresh Download (registerware) or Download Express (free/liteware) are worth a look.
http://www.freshdownload.com/ - be aware, the price is a spamvertising email every time they update something, even if it's NOT the one you registered - I belive it IS confined to just them though - not seen any other additional spam to the address I used.
http://metaproducts.com/mp/mpProducts_Detail.asp?id=18
Has additional features which terminate after 30 day trial - should continue to function (freeware mode) in the same way as previous versions