If you have limited space go for a midi tower - but you still don't want a Micro-ATX mobo in it - it really will limit you and you can get a P43 ATX for approx the same money if money was an issue.
Now would be a good idea for you to list the components that you like the look of and advise us on whether you're sticking with a socket 775 mobo or whether, by dropping the SSD, you're going to future proof your build with the i7. That way we can advise you more effectively.
As i mentioned before - remembering, that the spec you posted originally will run all games without breaking much of a sweat - if money is tight. (with a decent ATX mobo in it of course

- and with the SSD dropped you could upgrade to a 4890 which would see that spec bulldoze through games.)