Would I have enough space to upgrade to 11.04 from 10.10 on a 4gig Asus EEE? I have an SD card in it giving another 4gig. How can I use that space?
That depends on how much space you've used. This page on Ubuntu wiki (last update 2009) says bare minimum is 1.8 GB. However, most of that is not new, so an upgrade will probably not require a lot of additional space.
If you mount your SD as a system file directory, you can have system files on your SD. However if you remove your SD card, your system may stop working!
To move a system directory to your SD card, see this question. You can follow a similar process to move your home directory to your SD card.
To move your whole Ubuntu install to your SD card, see this question.
If you're not using Update Manager, see this question on setting mount points in the live-cd installer.
Keep in mind that your SSD drive and your SD card will have different performance characteristics. I'd suggest you ask a new question about how to layout your Ubuntu install with a small SSD and SD card.
Well, you will not have any problem with disk space during the install. Depends on how you will be using ubuntu whether for experimental purposes, etc. But if you look forward to handling all your day-to-day activities in ubuntu, then you should reconsider assigning the partition a larger disk size.
I started off with ubuntu with fairly a low-disk space but that has changed now because I need to install applications, store files etc.
just know that the 2GB can handle the installation but you might not enjoy it.