markw's right in that you should think of upgrading the front three speakers together. If your budget only allows for two speakers, then it's quite likely that you'll get better performance by going temporarily with a four speaker set (no center speaker). It's not quite as vital to match the surrounds to the mains, as it is to match the center to the mains, so you can get by with mismatched surrounds. Obviously, it's preferable to timbre match all the way around, but given the way that most movie soundtracks are mixed, it's easier to get away with mismatched surrounds. However, with multichannel music and an increasing number of newer movie soundtracks that aggressively use the surround channels and share a lot of sounds between the main and surround speakers, the importance of matching surrounds is much greater.

If the existing Take 5 center speaker poorly matches whatever new mains you choose, then it's fine and even preferable to go without a center speaker until you round up enough funds to add the matching center speaker. I would max out the budget for the best possible pair of main speakers you can find, and hopefully you'll be able to save up enough to get the matching center speaker soon.