ECNs do trade matching, which is very similar to bank transactions in that it requires synchronized, transactional processing. You've got huge lists of bids and asks that are matched as quickly as possible. I know engineers who work(ed) on Archipelago, now known as NYSE Arca. It's UNIX-based, first Sun and now Linux. There are no mainframes at all. These aren't particularly impressive boxes or a ton of them either. The software is fast and well architected.

NYSE also apparently unplugged their last mainframe in 2008[1]. It appears NASDAQ OMX's INET trading platform is based on x86-64/Linux/Oracle stack on commodity hardware[2]. NASDAQ's US exchange has a record of 194,205 orders per second[3]. The latest VisaNet peak message rate I could find was from 2008 and was 8,442 transactions per second[4]. Based on the fact that Visa aggregates transactions over hundreds of major banks, I would guess that each individual bank processes less credit card transactions. Visa's 2009 volume was 9.0 billion transactions[5]. I couldn't find anything substantial on NASDAQ's stock market order volume, but NASDAQ does about 10 billion contracts/year JUST in US & European option derivatives[6].

[1] http://searchdatacenter.techtarget.com/news/article/0,289142...

[2] http://www.nasdaqomx.com/digitalAssets/66/66992_genium_inet....

[3] http://www.nasdaqomxtrader.com/trader.aspx?id=inet

[4] http://investor.visa.com/phoenix.zhtml?c=215693&p=irol-n...

[5] http://www.creditcards.com/credit-card-news/credit-card-indu...

[6] http://www.nasdaq.com/aspx/company-news-story.aspx?storyid=2...