Anything that has less then 8 GB will not be able to run what you are planning to run, and even then it will be difficult. For Exchange 2010 you need at least a DC in your network and an Exchange server. A 2008R2 DC does not requier more then 512 MB if you just use a core install, which is a nice exercise anyway. Exchange 2010 has however the new HA feature called DAG. So to get that running you need a DC, Exchange with CAS and HUB and two servers with the mailbox role. If you are on a shoe string budget you might place it all on one server and forget about HA. For Exchange to run propperly in a test enviroment you need about 2 GB of RAM.
Also for building multiple forests, multiple domains, multiple sites, you need to be able to install several domain controllers.
A test server is very good investment if you go for your certifications. It will eventualy pay itself back. I remember I had a raise of 800 Euro per month after I got my MCSE and moved to a new employer. (I live in the Netherlands) So If you regard it as a real investment then scrape some money together and buy a real server motherboard that can use a lot of RAM. You don't need a full HP or DELL server but a server motherboard lets you install a lot more RAM then a normal PC motherboard. I have an ASUS motherboard and I can stick 144 GB of RAM in my server. (currently have 24 GB) This means I can use my server for many years to come. In the end it is more efficient to get your certifications fast then to safe money on a very limited server.