Just an FYI, if you have a loud keyboard it doesn't matter how good the desktop mic is, it's still going to pick up the keyboard loud and clear if it's closer to the mic than you are. I know you said you don't want to use the Modmic(or a mod similar to what hooli posted), but it really is the best option if you have a loud keyboard(IMO).
Also, people are right about the Zalman mic behaving 100% different if you have a sound card. It honestly sounds really good with my 5 year old Creative X-Fi, and I've heard some people who have the same mic who sound like they're in a fishbowl because they use some awful built in motherboard audio. To add to that, you'll likely enjoy your headphones a lot more if you get a sound card. Do what you want, but I would recommend either the Modmic, or a new sound card, or both. What I do not recommend is using a desktop mic with a loud keyboard because your teammates will not enjoy that one bit.