If you're just doing two hosts (or if you expect no more than a couple more to be addded in the forseeable future) you might want to save yourself the cost of the access point and run normal client cards in adhoc mode; this mode allows two normal cards to talk with each other, more can be configured with pairwise configurations. I run my home net that way.
I've no idea about Windows support, perhaps other folks here can comment on that. I'd recommend getting whatever 802.11b cards you can find for a good price, they all seem to be well supported these days and the interop problems seem to have died out. If you want to have the option of war-driving, go shopping for the software you want first, and then see what sorts of cards it supports. E.g. I like Kismet, and it has nice support for most cards; it works with both libpcap+Linux-Wireless (e.g. Aironet) and Wlan-NG (prism/2 chipset). Regardless of what cards you get, or whether or not you use an access point instead of using adhoc mode, you need to address security. As you'll find out when you start experimenting with war driving, 802.11b has no security at all. It's fairly comparable to being directly exposed to the internet; the threats are somewhat different, but for many purposes they're roughly comparable. In either case you have anonymous and practically untraceable attackers, armed with easy-to-use tools, going around twisting doorknobs looking for vulnerable nets. Each machine that has a wireless interface should be configured as a firewall; disable all unneeded services, and if you must leave any services running that aren't internet-grade secure (commonly, an X server) make sure you block it with packet filtering (ipchains or iptables for Linux, some sort of add-on "personal firewall" for Windows). The only protocols you should run over your wireless are internet-grade secure protocols. If you can confine yourself entirely to ssh, https, and the like, then you're done; if you need to run less-secure protocols over wireless, you'll want to set up a VPN for the purpose. There are a lot of VPN choices, but if you need interop between different OS platforms you're most likely to enjoy success with IPSec. There's another mailing list run here on securityfocus for VPNs, that'd be a good place to read up. -Bennett
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