Centos – Have I pushed the limits of the current VPS or is there room for optimization

I am currently on a mediatemple DV server (basic) 512mb dedicated ram, this is a CentOS based VPS with Plesk and Virtuozzo. My experience with it from day 1 has been bad and I only could sooth my server issues with several caching "Band-aids," but my sites are not as small as they were a year ago either so the issues have worsen.

I have 3 Drupal installs running on separate (plesk) domains, 1 of those drupal installs is a multisite, that consists of 5-6 sites 2 of those sites are bringing in actual traffic. Those caching "Band-aids" I mentioned are APC, which seemed to help alot initially, and Drupal's Boost, which is considered a poorman's Varnish, it makes all my pages static for anonymous users.
Last 30day combined estimate on Google Ananlytics: 90k visitors 260k pageviews.

Issue: alot of downtime, I am continually checking if my sites are up, and lately I have been finding it down more than 3 times daily. Restarting Apache will bring it back up, for some time. I have google search every error message and looked up ways to optimize my DV server, and I am beyond stump what is my next move. Is this server bad, have I hit a impossibly low restriction such as the 12mb kernel memory barrier (kmemsize), is it on my end, do I need to optimize some more?

*I have provided as much information as I can below, any help or suggestions given will be appreciated

Common Error messages I see in the log:

[error] (12)Cannot allocate memory: fork: Unable to fork new process
[error] make_obcallback: could not import mod_python.apache.\n
Traceback (most recent call last):
 File "/usr/lib/python2.4/site-packages/mod_python/apache.py", line 21, in ?
 import traceback
 File "/usr/lib/python2.4/traceback.py", line 3, in ?
 import linecache
 ImportError: No module named linecache
[error] python_handler: no interpreter callback found.
[warn-phpd] mmap cache can't open /var/www/vhosts/***/httpdocs/*** - Too many open files in system (pid ***)
[alert] Child 8125 returned a Fatal error... Apache is exiting!
[emerg] (43)Identifier removed: couldn't grab the accept mutex
[emerg] (22)Invalid argument: couldn't release the accept mutex

cat /proc/user_beancounters:

Version: 2.5
       uid  resource           held    maxheld    barrier      limit    failcnt
     41548: kmemsize        4582652    5306699   12288832   13517715   21105036
            lockedpages           0          0        600        600          0
            privvmpages       38151      42676     229036     249036          0
            shmpages          16274      16274      17237      17237          2
            dummy                 0          0          0          0          0
            numproc              43         46        300        300          0
            physpages         27260      29528          0 2147483647          0
            vmguarpages           0          0     131072 2147483647          0
            oomguarpages      27270      29538     131072 2147483647          0
            numtcpsock           21         29        300        300          0
            numflock              8          8        480        528          0
            numpty                1          1         30         30          0
            numsiginfo            0          1       1024       1024          0
            tcpsndbuf        648440     675272    2867477    4096277    1711499
            tcprcvbuf        301620     359716    2867477    4096277          0
            othersockbuf       4472       4472    1433738    2662538          0
            dgramrcvbuf           0          0    1433738    1433738          0
            numothersock         12         12        300        300          0
            dcachesize            0          0    2684271    2764800          0
            numfile            3447       3496       6300       6300       3872
            dummy                 0          0          0          0          0
            dummy                 0          0          0          0          0
            dummy                 0          0          0          0          0
            numiptent            14         14        200        200          0

TOP: (In January the load avg was really high 3-10, I was able to bring it down where it is currently is by giving APC more memory play around with)

top - 16:46:07 up  2:13,  1 user,  load average: 0.34, 0.20, 0.20
Tasks:  40 total,   2 running,  37 sleeping,   0 stopped,   1 zombie
Cpu(s):  0.3% us,  0.1% sy,  0.0% ni, 99.7% id,  0.0% wa,  0.0% hi,  0.0% si
Mem:    916144k total,   156668k used,   759476k free,        0k buffers
Swap:        0k total,        0k used,        0k free,        0k cached

MySQLTuner: (after optimizing every table and repairing any table with overage I got the fragmented count down to 86)

[--] Data in MyISAM tables: 285M (Tables: 1105)
[!!] Total fragmented tables: 86

[--] Up for: 2h 44m 38s (409K q [41.421 qps], 6K conn, TX: 1B, RX: 174M)
[--] Reads / Writes: 79% / 21%
[--] Total buffers: 58.0M global + 2.7M per thread (100 max threads)
[!!] Query cache prunes per day: 675307
[!!] Temporary tables created on disk: 35% (7K on disk / 20K total)