You have some problem other than Parallels. As the other said, is that 1 GB RAM total, or 1 GB allocated to your Parallels VM? Actually...either way, it should be enough. I don't want to blame it on Parallels, since I know most people don't have trouble with it, but there's something else going on here if it's running that slow for you. I have a 2 GB MacBook with VMWare Fusion. I gave my Windows XP virtual machine only 512MB RAM, and it still flies. Faster than my last PC laptop, even.
I disagree with 1 gig of ram being enough to run parallels well.
I have a friend who upgraded his MBP to 2 gigs of RAM recently. He noticed a significant boost to parallels performance.
If you take a look at this thread:
https://forums.macrumors.com/threads/342278/ , you'll see that most people recommend at least 1 gig for the typical user.
But that's BEFORE you split that in half with parallels (not that you have to use half, but you know what I mean).
If your mac flies with 1 gig of ram with typical use, then it will probably fly noticeably faster with more of it.
If you really want an accurate assessment though, there are two things you should look at.
Note: you can do both of these in Activity Monitor, but I recommend you download the iStat Pro widget as it makes it very easy to periodically check your systems stats.
1) Take a look at how much available RAM you have. Not just how much FREE ram you have though. You want to look at inactive+free. Inactive RAM is not really being used by anything right now, but still contains the information it was being used for.
For example, if you open up an application and close it, whatever RAM that application used will be labeled now as inactive. If you then reopen that application, it should open more quickly because instead of having to load the info back into RAM from scratch, it can just use what was left in the inactive RAM. If the system is running low on RAM, then it can easily clear the inactive RAM and use it for whatever task it deems necessary.
2) Take a look at your page ins/outs. I don't know what number you want there though, but generally a lower number is better. The amount of paging though will also depend on how long the machine has been running since the last restart.
If your available ram is very low of if you have lots of paging, then you'll probably benefit from more RAM.
If you're playing any games that are require more performance than a typical game of tetris, then you should probably get more RAM regardless.