Apache Bench: Why so many 503s?
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I'm running a dedicated server on DreamHost: Quad Core, 4GB RAM.

Linux (Debian), Apache, PHP (with XCache for opcode caching), MySQL.

The URL I'm querying has 18 MySQL queries every time.

I'm wondering why when I do "ab -n 500 -c 50 http://<*url*>/" I'm getting the following results: 

    Server Software:        Apache
    Server Hostname:        <url>
    Server Port:            80

    Document Path:          /
    Document Length:        162809 bytes

    Concurrency Level:      50
    Time taken for tests:   12.664 seconds
    Complete requests:      500
    Failed requests:        377
       (Connect: 0, Receive: 0, Length: 377, Exceptions: 0)
    Write errors:           0
    Non-2xx responses:      377
    Total transferred:      20291876 bytes
    HTML transferred:       20147278 bytes
    Requests per second:    39.48 [#/sec] (mean)
    Time per request:       1266.372 [ms] (mean)
    Time per request:       25.327 [ms] (mean, across all concurrent requests)
    Transfer rate:          1564.81 [Kbytes/sec] received

    Connection Times (ms)
                  min  mean[+/-sd] median   max
    Connect:        0    0   0.4      0       2
    Processing:   194 1190 1617.0    248    6060
    Waiting:      194  965 1263.9    248    5190
    Total:        194 1190 1617.0    248    6060

    Percentage of the requests served within a certain time (ms)
      50%    248
      66%    359
      75%   1555
      80%   2787
      90%   3979
      95%   4709
      98%   5427
      99%   5967
     100%   6060 (longest request)

All 377 failed requests are 503s.  What could be causing this?  Running `top` while doing this saw my processor spike to about 24%, but didn't tax it too much.  

My CMS says that with a single load of the page, it takes .0017 seconds to load base classes, .1711 seconds for controller execution time, for a total execution time of .1731 seconds, using 2.887 MB of memory.

`ab -n 100 -c 10` always has 0 failed requests, but it seems that anything with concurrent connections over ~25 has a bunch of failed connections.

I'm expecting a ton of traffic next week all at once, how can I best prepare my server?  Will this hold up as is?