If you are getting a 404 from your old hosting company, your DNS isn't resolving to Dreamhost yet. You might want to check to make sure everything is hunky-dory with your domain registrar.
Assuming your domain is TheMartian.com ---
Open an MSDOS window, and type "tracert TheMartian.com".
This will let you see what the domain name resolves to - whether you are going to Dreamhost or not. Assuming that you did everything correctly at your registrar, it can take 2 or 3 days for your domain to resolve correctly - or as long as a month, if you have an especially bad ISP.
If your DNS isn't resolving itself properly, you can still access the DreamHost version of your site. Copy the file C:\windows\hosts.sam to c:\windows\hosts and edit it. The lines that start with a crosshatch are commented out. What you want to have are several lines that are NOT commented out, which say
192.168.1.1 TheMartian.com www.TheMartian.com
except that instead of those 192.168 addresses, you should have the IP address listed in your control panel. (click on domains, then DNS). The 127.0.0.1 address does NOT get changed.
OK, so once this file is in place, it starts to work immediately. No need even to reboot.
Now, you can see whether you are uploading your files in the right place.
Click on domains, then manage, then on the line that says "web" in the "service" category, click on "Edit"
You probably want the "a fully hosted domain" radio button checked. The "Web Directory" line shows where you should put the index.html file which is your home page.
Since you have the hosts file installed, you should be able to FTP to your domain and stuff an index.html file there, then see it immediately in your browser. (No need to wait for the hourly server refresh.)