mac networking help for a recent pc convert and networking rookie

Hey all!
I hope someone here can give a quick hand. I got a felling I'm doing something a bit daft and it's probably a simple fix, but it's taking me hours without any luck at all...
I made the switch to mac about three months ago buying a macbook which I love. I started using it as a media centre, which was great, until the girlfriend wanted to watch tv while I wanted to play on the laptop. So I bought a mac mini as a dedicated media server. However, I can't seem to get both the laptop and the mac mini connected to the net via my sitecom wireless router t the same time. One or other will work, but not both at the same time.
Here's my settings (I don't know anything about networking, so bear with me) ...
The router is set to manually assign IP addresses (i think?!) ie not using DHCP (I'm hoping I'm not making an arse of myself here). The IP address of the router is 192.168.0.1 and I can connect to that to configure it. I have opened up a range of IP addresses - 192.168.0.2 to 192.168.0.3 - and have turned off DHCP.
On the mac mini, in the tcp/ip bit, I'm configuring ipv4 manually, have the ip address as 192.168.0.2, subnet mask as 255.255.255.0, router as 192.168.0.1, and everything else is blank. The mac mini works fine and I get internet via wireless.
On the laptop, it's the same apart from the ip address which is 192.168.0.3 and it doesn't work - in network diagnostics i get green for airport, airport settings, network settings, isp, but red for internet and server.
I've also tried using DHCP, but with that I get the same thing - only one machine will connect.
Any ideas?
Thanks very much. You are my last hope before calling in the eperts and shelling out tons of cash.
Thank you very much in advance for any suggestions.
Evade