Most space can be reserved through space@cc, but there are some things that can't, and Paces is one of those. So if you want to have a Paces party, you'll have to talk to whoever's in charge of that, which will be a student. You'll probably have to put down a deposit for Paces, but you should get your money back without any trouble.
For a party, you may want music. If there happens to be a d.j. in SWIL, you're in luck, but, if not, you'll have to find one elsewhere. There are usually several people on campus who d.j., so you should find one or two and talk to them. If you have a specific music theme in mind, try to be insistent about it (unless it's a really bad theme). We tried to have an eighties music theme, and achieved some success, but it turned into techno after awhile.
Your d.j.'s will probably be affiliated with Rattech, which is the student sound equipment organization. They will likely have a certain amount of lighting equipment too, which they will bring, and tell you about in advance so you can plan accordingly. As far as homespun decorations go, decide on a theme, but try to get SWILfolk to contribute stuff. You can get amazingly far on things SWILlies have in their rooms, supplies from halloween stores, and random objects from Home Depot.