Tom asked in a comment to the last post for a few notes on the various editions to Calvin's Institutes of the Christian Religion. I'm only going to reference the four editions available from the WTS Bookstore because anything else you can find either isn't in English or is probably too expensive to be worth looking at.
The main editions that are available are the Beveridge translation and the Battles translation edited by John McNeill. Both of these are good. The main differences are general readability and the Scripture references. The Battles version is a better English translation in that it's a lot easier to read. The language is closer to modern English and you can get through it very quickly. McNeill also provides some footnotes describing opponents that Calvin occasionally references. The main problem is in the Scripture references (more on that to follow).
The Beveridge translation is an older one and so it does not read as easily as the Battles one. The main advantage is that unlike McNeill/Battles, Beveridge did not add Scripture references into the text. When Calvin wrote his Institutes he intended for them to be read along with his Commentaries. When you see a Scripture reference in the Institutes it was meant to serve as a sort of footnote so that you couldthen turn to the Commentaries on that passage and see the exegesis that lies behind Calvin's systematic presentation of the teaching of Scripture. This really illustrates how strongly Calvin's work in systematic theology is tied to his Biblical exegesis and how that has been the Reformed tradition following in his footsteps. The problem with the McNeill/Battles edition is that they add references to the text but do not note which references are original and which are editing. This makes it more difficult to move back and forth between the Institutes and the Commentaries as Calvin intended.
The other editions available are the 1536 edition (this is the first edition that Calvin wrote) and the 1541 French edition (Calvin himself translated his work from Latin to French several times so that it would be available to laity). These editions are really mainly only of advantage to people who want to look at the development of Calvin's thought over about 20 years between the first and final editions. One of the truly interesting things about Calvin is that unlike Luther, Melanchthon, Zwingli, and Bullinger he never changed his mind about any significant doctrinal point. The major differences lie in the order of presentation and what he expanded upon over the years.
In the end you really can't go wrong with either Battles or Beveridge. Personally, I think that it is worth the money to have both the Institutes and the Commentaries if you have the option. You ought to be able to find a package with all of the Commentaries and the Beveridge edition together for no more than $120. I think that's a great bargain and worth the price. If that's too much money for what is budgeted towards books then the Battles edition is an easier read if all you have is the Institutes. Beveridge is still a lot cheaper but you'll find Battles to be better going. One other thing that I would recommend with any option is the recent Theological Guide to Calvin's Institutes: Essays and Analysis. This is an extremely helpful resource on Calvin's theology. While it shouldn't replace reading Calvin it can be used as a great supplement and many of the writers do reference the Commentaries and other writings in their essays.
As an aside to this, I wanted to point people to a recent post by Wes White, a PCA pastor. Pastor White recommends a few systematic theologies that are pretty foundational from all of the options out there. All of his recommendations are very good though I probably wouldn't put Hodge quite as high just because of how much Scottish Common Sense Realism affects his prolegomena [first things] (it is rather interesting that Hodge is word-for-word with Turretin almost everywhere but in the first volume). I would also say that if you only ever have one Reformed Systematic Theology then you want either Herman Bavinck's Reformed Dogmatics or Francis Turretin's Institutes of Elenctic Theology. If you can only get a one volume work then Van Genderen and Velema's Concise Reformed Dogmatics that is only recently available in English is probably your best bet. Finally, possibly in place of any of these (again, if you're limited if what you can get) I would recommend John Murray's Collected Writings. While Murray never wrote his own systematic theology (he worked from Hodge and Turretin at WTS) he covers nearly all of traditional categories in various essays and articles included here. I find Murray very lucid and you should enjoy his careful exegetical theology. Murray is also very sensitive to redemptive history, having been a student of Geerhardus Vos at Princeton.