1) Oracle 10gR2 Express Edition (XE) is built using the same kernel as Standard Edition and therefore follows the same basic limitations as found in the Reference manual for 10gR2 (see http://tahiti.oracle.com and click on "Oracle Database documentation, 10g Release 2 (10.2). " link - the Reference manual is on the first page that shows up)
2) XE imposes additional limitations as described in the License documentation found at "Oracle Database Express Edition documentation, 10g Release 2 (10.2)." (again, starting from http://tahiti.oracle.com). The feature limitations and includes are spelled out in that doc, as are the physical limits - for your convenience they are
"Any use of the Oracle Database Express Edition is subject to the following limitations; 1. Express Edition is limited to a single instance on any server; 2. Express Edition may be installed on a multiple CPU server, but may only be executed on one processor in any server; 3. Express Edition may only be used to support up to 4GB of user data (not including Express Edition system data); 4. Express Edition may use up to 1 GB RAM of available memory."
Note that setting the Open Cursor (and PRocesses, etc) has an implication related to the memory, and setting these too high will unnecessarily chew up parts of your 1GB memory limit.